In-Game Screenshots Only!
All pictures must be widescreen or square. Pictures with more height than width will have the bottom cropped off.
The gallery will not accept pictures larger than 2MB. It will give you an error saying "you have exceeded the file size limit". Unfortunately, it will not tell you which image was too large, so figure it out yourself. It will also prevent you from uploading further pictures until you restart your browser. If you wish to compress your image into a smaller size, try a site like http://optimizilla.com/ (might decrease the quality of the image).