I think the problem in your case may be that there are no slide images at the top level of the album - everything is in folders. So, the standard routine for creating the profile page listing can't find an image to use. It can see only the thumbnails.
Your guess is correct. There are only thumbnails to be found in the root. The thumbnails in that album are also smaller (90x60) than the default for Turtle (200x150), so that's why the album icon is worse than usual.