A couple of users on the Apple discussion boards have described a small problem some folks may have with Preview not loading pictures in its main window under Snow Leopard. When images are double-clicked, the program will open and they will show in the sidebar, but the main viewer remains blank.
This seems to be a problem with the Preview application itself, and not with the core libraries and other system features that Preview uses to render images. Specifically, it appears to be a bug with the 64-bit coding of the program, since it functions just fine for affected users when running in 32-bit mode. As such, an easy workaround for this until Apple releases an updated version of the program is to force it to open in 32-bit mode, which can be done by checking the "open in 32-bit mode" box in the information window for the application.
I tried replicating this problem on my MacBook Pro running OS X 10.6.1, but images of various formats load just fine in both 32-bit and 64-bit modes on my machine. I suspect that most people are able to run Preview just fine, and while one possibility could be that this is a hardware-specific bug, it seems more likely that it is due to corruption in the binary or in the application's settings. As such, reinstalling it using Pacifist and removing its preferences files may also help.
Pacifist is available from the developer's website, and can be used free of charge to locate and install the "Preview.app" package off of the Snow Leopard DVD. To be sure the old installation is replaced, move the current application to the trash before installing the replacement, and then empty the trash only after a successful installation. The preferences file is called "com.apple.Preview.plist" and is located in the /username/Library/Preferences/ folder, so be sure to remove that as well.