Problem with Image

Hi, I usually modify my own blog and had no problem with it. But for some reason, when I like to upload my own custom header in any theme that I like I have a problem uploading it! It just wouldn’t show up! I click upload ( yes it’s all the right width & height) but it wouldn’t show in the preview.. it’s just blank, and can’t even do with cropping because I can’t see it. When I click Media/Library there’s no preview in the thumbnail. You can click edit image and it will show but I can’t use it as a header! I tried it in every theme I want but just couldn’t use the header… ?? please help.