Hello [name]! Thanks for joining the GMC. Before making any posts in the Tech Support forum, can we suggest you read the forum rules? These are simple guidelines that we ask you to follow so that you can get the best help possible for your issue.
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.