The uploading feature for pants and shirts only accepts .png files, not anything else. Not even the almost-identical .PNG format, or the similar .JPEG, .JPG, or other files.
Not everyone can do .png files, you know.
File types are too easy. Image formats are even easier, to the point where cavemen if they had technology they can do it. Simply slap that picture on Paint.net, save it as a png. TADA