I've installed WordPress successfully in a new site. In the Presentation page of the Admin section, preview screenshots of the default themes show up as expected. However, I FTP'd a dozen or so of the WordPress Themes Contest winners (after unzipping them on my hard drive) and none of these present screenshot previews in Presentation.
The new themes are in the proper folder (same as the default themes) and are set to 755 in Permissions, as are the default themes.
Any idea what's wrong?