Test an icon from a previous game that you KNOW worked previously. If it still gives that error then it would suggest there is a bug... or maybe the specs have changed (in which case it should be documented). If an image that previously worked now doesn't, then make a zip of the images and a YYZ of a project to show the issue, and file a bug with YYG. If a previous image DOES work, then there's something wrong with the image you are trying to use.