When uploading a JPG under 15kb with the correct pixels into the background section of custom themes in org settings it doesn't accept the image even though its under 1366 x 50 pixels and also under 15KB. Also is a JPG.
@AB21805 I am having the exact same issue. I found that it will upload PNGs but not JPGs. And a PNG under 15KB is too low rez to be usable.