post thumbnail not working

I’ve tried several magazine style themes that support the usage of post thumbnails, however none of them work. All I get with the posts is the plain text, and, depending on the theme, just the blank area where the thumbnail was supposed to appear. This happens even with a proven theme that worked fine on my wordpress.COM blog.

Is it something that has to be activated within the wordpress code? I’m using the latest version of wp.

Please help.