In the latest version, 1.9.2 (probably in others as well, there is a small bug in admin/manage-images.php, lines 464 and 465. Because you are using the esc_url function, in case the $picture->imageURL or $picture->thumbURL contains a space character, it gets stripped so it won't show the image thumbnail in the admin when managing a gallery and it will throw warnings in the error log. Just try to add a picture with spaces in it's name in a gallery and you should be able to replicate the problem.
Perhaps the imageURL and thumbURL class members need to be URL encoded when they are populated, but maybe this breaks something else... I am not sure where is this code located, though...