At least 820 pixels wide by 312 pixels tall. Anything smaller is rejected when you try to upload it.

That is a ratio of roughly 2.6 to 1, so a wide landscape image rather than a square one. A good target is 1640 by 624, which is the same shape at twice the size. Use JPG or PNG.

The same minimum applies to the cover photo on your organization page, on your opportunities and on your forms.

A category cover photo is saved to your cover photo theme library, so one image can be reused across several categories. It is also saved as a JPEG once you crop it, which means transparency is lost. Use a photograph there rather than a transparent graphic.

For the full picture, including logos, profile photos and every other place you can upload an image, see Cover Photo and Logo Specifications on Vome.