This is an odd case, but it should work. If I create an album that has only a few PDF's in it, and no regular JPG's, and upload it to jalbum.net, it fails to create an entry for the album on my profile page. The console error is:
java.io.IOException: Error while adding album. Server reported: Album thumbnail hasn't been specified.
The album actually does get uploaded, and I can go to my profile page and add it manually (as if it were an external album).
If there's no thumbnail image available, the album should be added to the profile page with the standard "no image" placeholder.