Brand Kit for Newsletter Writers: Substack, ConvertKit, and Beehiiv Headers
Quick Answer
Newsletter writers need two core image assets: a square publication logo (upload at least 512×512px, used as the favicon, sender icon, and app icon across Substack, ConvertKit, and Beehiiv) and a wide header or cover image built at a 2:1 ratio, roughly 1200×600px and exported at 2x for retina screens. Design the header at that ratio regardless of platform, since email clients render around 600–650px wide no matter how large the source file is. Keep the file size down, ideally under a few hundred kilobytes, because a heavy header image is often what triggers Gmail's clipped-message cutoff and slows the newsletter's load in the inbox. Uploading a square logo into a wide header slot, or a wide banner into a square logo slot, is the most common mistake and usually results in a stretched or cropped image on one of the three platforms.
A newsletter writer moving between Substack, ConvertKit, and Beehiiv runs into the same small problem three times: each platform wants a differently shaped image in a slightly different slot, and none of them will tell you your logo looks wrong until after it's already live and slightly blurry in someone's inbox.
None of this is complicated once you know what each platform actually asks for. The annoying part is that nobody collects it in one place, so you end up guessing, or re-exporting the same file four times because the first three didn't fit.
Why the same logo keeps looking wrong
Two different image slots get confused for one another constantly: the square logo (used as a favicon, an app icon, and the small circle next to the sender name) and the wide header or cover image (used at the top of a post, a landing page, or an email).
Drop a square logo into a header slot and the platform stretches it into a wide rectangle, distorting anything that isn't perfectly abstract. Drop a wide banner into a logo slot and it gets center-cropped into a square, usually cutting off half a wordmark. These two files are not interchangeable, and every platform in this post asks for both.
There's also a rendering mismatch specific to newsletters that doesn't show up on social platforms: your header image gets displayed twice, once on the web version of the post and once inside an email client, and email clients cap the effective width at around 600 to 650 pixels regardless of how large the source file is. Design with that constraint in mind, then export larger for the pixel density on modern screens, and the image holds up in both places.
The exact numbers below are what each platform's upload screen asks for as of this writing. All three tweak their editors often enough that it's worth a quick glance at the current upload prompt before assuming these haven't shifted.
Substack
| Asset | Recommended upload | Where it shows up |
|---|---|---|
| Publication logo | Square, at least 512×512px, PNG or JPG | Favicon, app icon, next to the publication name in the app and in email |
| Cover/header image | Landscape, roughly 1200×600px (2:1 ratio) | Top of the publication homepage, and reused as the link-preview image when a post is shared |
Substack has generally reused the cover image for social sharing, so there's usually no separate OG image to manage. Check the link preview after publishing to confirm it pulled the right one. The logo, by contrast, gets used at very small sizes, in the app sidebar and the browser tab, so a wordmark with fine detail will turn into a smudge. A monogram or a simple icon mark holds up better than a full name lockup at that size.
ConvertKit
| Asset | Recommended upload | Where it shows up |
|---|---|---|
| Creator profile photo | Square, roughly 400×400px | Sender identity, creator profile, some landing page templates |
| Broadcast/email header image | Design at 600px wide for email rendering; export at 1200px (2x) for retina | Top of broadcast emails and landing pages |
ConvertKit gives more layout freedom than Substack or Beehiiv, since broadcasts are closer to a flexible email builder than a fixed template. That flexibility is also where things go wrong: it's easy to drop in whatever image size is on hand and let the builder auto-scale it, which is how a 400px-wide image ends up soft and pixelated after "fitting" a 1200px-wide header block. Export at the pixel size the block actually needs, not the size that happens to look right in the editor preview.
Beehiiv
| Asset | Recommended upload | Where it shows up |
|---|---|---|
| Publication logo | Square, at least 600×600px; transparent PNG for the nav bar | Site navigation, footer, app icon |
| Post/email header image | Roughly 1200×600px (2:1 ratio), exported at 2x (2400×1200) for sharpest rendering | Top of the post and the email version, and it typically feeds the social share card too |
Beehiiv's navigation bar sits on whatever background color the theme uses, and themes vary a lot more than Substack's fairly fixed layout. A logo with a white box baked into the file can show a visible rectangle against a colored nav bar, so export it as a transparent PNG and check it against both a light and a dark theme before publishing.
The one social-share size worth knowing beyond all three
Outside these platform-specific slots, there's a fourth, more universal size worth having ready: the standard Open Graph image, 1200×630px, is what Facebook, LinkedIn, and most link-preview tools fall back to whenever a platform doesn't generate its own share card. Substack and Beehiiv both tend to pull their previews from the header image already in use, so a dedicated OG file mostly matters as insurance for anywhere those defaults don't apply, like a personal site linking back to the newsletter.
What actually goes wrong
The failure that shows up most often isn't a wrong pixel count. It's file weight. A 4MB header image loads slowly inside an email client, and in Gmail specifically, an email that crosses roughly 102KB in total size gets clipped behind a "view entire message" link. The unsubscribe footer sits at the bottom of most email templates, which is exactly what gets cut off first when a heavy header image pushes the message past that limit. Compress the header image before uploading. If a 1200×600 banner is landing in the megabytes after export, the file is almost always carrying more color depth or embedded metadata than a header banner needs, and running it through a compressor should shrink it drastically without a visible quality loss.
The second common mistake is treating the three platforms as interchangeable. A creator moving a newsletter from ConvertKit to Beehiiv, or cross-posting the same header to Substack, often just re-uploads whatever file is already sitting on their desktop. If that file was sized for ConvertKit's flexible layout, it may be too narrow for Beehiiv's fixed 2:1 slot, or too heavy for fast email rendering either way. Keep the source files organized by shape, square logo and wide header, rather than by platform, and each new platform just needs the right one pulled from the folder.
A short brief for generating both files
Publication name:
Format needed: square logo (icon/monogram) and wide header (2:1 banner)
Where the square version needs to work: favicon, app icon, sender avatar
Where the wide version needs to work: post header, email header, link preview
Background: transparent PNG for the square logo; solid or on-brand background for the wide header
Tone: what the newsletter is about, in a sentence
That's really only two files: a square icon-style mark and a wide 2:1 banner, each exported large enough to downscale cleanly. Start a logo on the Brandize home page, export the SVG, and both the square and wide versions can be generated from that one source instead of resizing a raster file by hand each time a new platform slot shows up.
A full brand kit covers more ground than these two images, an email signature for replies included. Podcast hosts and YouTube creators run into a similar two-asset problem with their own platform quirks, and the podcast brand kit guide and the YouTube channel branding guide cover those.
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