Every Reddit image size
Reddit uses the same banner size for profiles and communities, and the same square for your avatar and a community icon, so four sizes cover almost everything.
Profile banner
1920 × 384 px, 5:1 Across the top of your profile.
Community banner
1920 × 384 px, 5:1 Across the top of your subreddit.
Avatar / community icon
256 × 256 px, 1:1 Your profile picture, or your subreddit's icon. Shown as a circle.
Post image
1200 × 628 px, 300:157 A landscape image for a post.
Designing for a 5:1 banner
At 5:1, the Reddit banner is wider than almost any photo. A normal landscape photo keeps only a thin band from its middle. Pick an image where that middle band is the interesting part: a horizon, a row of products, a pattern.
Reddit shows banners on everything from wide desktop windows to phones, and narrower screens can trim the sides. Keep any text or logo near the center of the strip rather than at the edges.
For a subreddit, the banner and the community icon are seen together. A plain banner with a strong icon is easier to recognize than two busy images competing.
Images that suit Reddit posts
Reddit communities tend to prefer images that look made for the post, not for an ad. A clean crop at the right size, without watermarks or heavy borders, fits in better in most subreddits.
Each subreddit sets its own rules about images. Check them before posting, especially for anything promotional.