Drop in an image to check its dimensions, file size, and colour mode against Tumblr's real limits — then resize it to the dashboard width, full quality, a header, or an avatar. Nothing is uploaded.
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Tumblr accepts RGB only and rejects CMYK outright. CMYK files look completely normal in most viewers, so this failure is invisible until the upload dies. We read the colour mode from the file's bytes.
540×810 for the dashboard, 2048×3072 for the clickthrough version, 20 MB maximum. Taken from Tumblr's Help Center and cross-checked against what its CDN actually serves.
Resize to the dashboard width, full quality, a 2048×1152 header, or a 128×128 avatar. Square and 16:9 targets centre-crop instead of squashing.
Parsing and resizing both happen on your device using the File API and canvas. Your image never touches a server — ours or anyone's.
| Where | Dimensions | Max size |
|---|---|---|
| Post image (dashboard) | 540 × 810 | 20 MB |
| Post image (clicked) | 2048 × 3072 | 20 MB |
| Blog header | 2048 × 1152 | 10 MB |
| Avatar | 128 × 128 | 10 MB |
| GIF | 540 wide | 10 MB |
Verified against Tumblr's Help Center on 12 August 2026, and cross-checked against the sizes Tumblr's own CDN serves. Formats for static images: JPEG, PNG, WEBP. RGB only — CMYK is rejected.
Search for Tumblr image dimensions and you'll be told 1280 pixels, over and over. It isn't a Tumblr limit. It appears nowhere in Tumblr's documentation, and it isn't a ceiling on anything.
What actually happens is that Tumblr generates nine versions of every image you upload — 75, 100, 250, 400, 500, 540, 640, 1280 and 2048 on the long edge — and serves whichever fits the context. 1280 is just one rung on that ladder. The numbers that matter are the two Tumblr documents: 540×810 for dashboard display, and 2048×3072 for the larger version behind a click.
This is worth caring about because the stale number is conservative in the wrong direction. If you resize to 1280 thinking it's the maximum, you're throwing away detail that Tumblr would happily have served at 2048.
Tumblr recommends 540 pixels wide by 810 tall for how images display on the dashboard. When someone clicks an image, Tumblr serves a larger version up to 2048 by 3072. So anywhere between 540 and 2048 on the long edge is sensible: below 540 looks soft, above 2048 is downscaled and just wastes file size.
That number is widely repeated but it isn't a Tumblr limit. Tumblr's own CDN generates nine sizes for every image — 75, 100, 250, 400, 500, 540, 640, 1280 and 2048 on the long edge — and 1280 is simply one of them. The documented figures are 540×810 for dashboard display and 2048×3072 for the larger clickable version.
The most common invisible cause is CMYK colour mode. Tumblr accepts RGB only, and a CMYK JPEG looks completely normal in Preview, Finder, and most browsers — so there's no clue until the upload fails. Print-oriented exports from Photoshop and InDesign are frequently CMYK. Drop the file here and we'll tell you outright — and running it through any of the resize options converts it to RGB, so it doubles as the fix.
For static images: JPEG, PNG and WEBP, up to 20 MB. Note that some guides still list BMP, which the Help Center no longer includes, and omit WEBP, which it does. GIFs follow a separate set of rules with their own size thresholds.
The avatar is 128×128 square, up to 10 MB. The header is 2048×1152 — 16:9 — also up to 10 MB. Both are one-click targets here, and both centre-crop rather than distorting your image to fit.
Ten from the mobile apps, thirty from the web. That difference catches people out when they build a photoset on desktop and later edit it on their phone.
Scaling down is fine and is what you want — it discards pixels Tumblr would have discarded anyway. We never upscale, because enlarging a small image adds no real detail. PNG sources stay PNG so transparency survives; other formats export as JPEG at high quality.
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