Thumbnails come up as gray boxes

I am using the Unstandard theme. When I first installed the theme the images were a grayish pattern. I did some research and found out how to use the custom fields feature to specify the location of the thumbnail image. Now the thumbnails appear as blank gray boxes instead of having a pattern. If I intentionally put the wrong info into the custom field the boxes return to the gray pattern like they had originally.

I have tried 4 different images. The images I have modified to be web-friendly (3 jpgs and 1 gif). None of the images are over 50k and all are the size required by the theme. I have beat my head against the keyboard for a week trying to get this thing to work. I have completely uninstalled and reinstalled wordpress from my site. I have uninstalled and reinstalled the theme as well. I have removed all plug-ins except akismet and IntenseDebate.

The only thing I can think of is that the theme I downloaded needs a thumbnail viewer of some sort that it did not come with. Any help would be very appreciated.