This is what it says when you visit the Gallery upload page. Follow this and you'll be fine. Art Gallery Art images should be GIF or JPG format only. The full image should be between 100 x 100 and 4000 x 4000 pixels and be less than 512 KB in size. The thumbnail image (if supplied) should be between 50 x 50 and 200 x 200 pixels and be less than 15 kb in size.