Reddit ad image size: 1080x1080, 3 MB, JPG or PNG

Reddit's image ad is a JPG, PNG or GIF up to 3 MB. Its mobile list has 1080x1080 (1:1), 1080x1440 (3:4) and 1080x1350 (4:5), plus 1440x1080 (4:3) and 1920x1080 (16:9); its desktop list has the last two. Reddit says all ratios are accepted, but one it does not list may cause formatting discrepancies such as letterboxing. Details are from Reddit Ads Help, read 2026-09-29.

What are Reddit's image ad sizes?

What file type and size does a Reddit image ad take?

The image file is JPG, PNG or GIF with a maximum of 3 MB. The headline allows 300 characters, with 100 or fewer recommended. Reddit's page separately lists a thumbnail, which is only shown in conversation placement and compact view, and which it generates from the main image unless you upload another.

WebP is not on Reddit's list. That matters if a tool defaults to it: the sample response in Sume's Images docs shows an image/webp file.

How do I ask Sume for a Reddit ratio?

POST /v1/images takes a normalized aspect_ratio (the docs list 1:1, 4:3, 3:4, 4:5 and 16:9 among others) and a resolution tier of 512, 1K, 2K or 4K. output_format takes png, jpeg, webp or svg, so send jpeg or png for Reddit. A model accepts only the values its catalog lists, so read its supported_parameters first.

curl -X POST "https://api.sume.com/v1/images" \
  -H "Authorization: Bearer $SUME_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "bytedance-seed/seedream-4.5",
    "prompt": "a red panda astronaut floating in space, studio lighting",
    "aspect_ratio": "1:1",
    "resolution": "2K",
    "output_format": "jpeg"
  }'

Which ratio should I pick for a feed ad?

Reddit's own table puts 1:1, 3:4 and 4:5 under Mobile and gives desktop only 4:3 and 16:9, and it recommends one image for multiple devices, placements and view modes. So the choice is about where most of your audience sees the ad, not about a single correct size. Reddit also lists 1:1 and 3:4 under "Acceptable ratios". Reddit does not say which performs better, and neither does this page.

Will the file be exactly 1080x1080?

Do not count on it. The docs say to give a tier and a ratio, not pixels: size is shorthand for a tier, and explicit pixels are not served in v1. So a 1:1 request gives you the right shape at a size the model picks. Reddit calls its sizes recommendations, not limits, and a matching ratio is what protects you from letterboxing. Open the result and check its width and height and its file size against 3 MB before you upload; if you need exact pixels, resize the download in your own editor.

The ratio question is the same one video ads raise, and 4:5 vs 9:16 by platform covers the trade-off there. For how the image API handles ratios and custom sizes, see image API aspect ratios and custom sizes.