I was having trouble with images constantly switching places. For example: searching for tourism ship I stumbled in this image:
But it kept appearing as another image. After a few trials I noticed that the problem is that the image name was null, the filename being just ".jpg" or "-150x150.jpg".
I tried using a plugin that renames files to a hash of themselves after upload (using sanitize_file_names filter) but it seems that photodropper isn't calling that function.
I can solve temporarily the issue by renaming my filenames after upload or choosing another image, but it's only a temporary fix and it's annoying.