Thank you ch€ri{Bi}² for discovering and reporting this bug.
If something went wrong while deleting an image in ACP (Admin Control Panel) it will show a "success" message instead of "error".
To fix that find in admin/images.php (note, this is only a part of the line):
$error_log[] = "<b>".$lang['image_delete_success']
Replace it with:
$error_log[] = "<b>".$lang['image_delete_error']
P.S.
This bug only affects default, unmodified admin/images.php