Application thumbnails cannot have space in their names
Think I might have come across a minor bug (in Sense server 1.0.2).
If you upload an image to the Default content library, and the file name of such image contains a space, that space is converted into %20, and when you try to use the image for an app thumbnail it just shows a broken link.
Using underscores instead of spaces in the file name avoids the problem.
Having spaces in the path where the image file is stored is no problem, btw.