Image Duplicates

I have two questions and both are about using images on a wordpress site. I have searched the forums but have not seen any advice specifically addressing my issues.

I use wordpress to run websites for the college classes that I teach. Several assignments involve having students upload photos. This most recent semester there was some confusion about this process in one of my classes. As a result I had a number of students upload multiple copies of the same photo.The problem is that these duplicates are all “attached” to a post so I can’t just filter and delete by “unattached” in the media library. While they are “attached” to a post they are not actually inserted or used into the post. With about 60 students this ended up meaning a lot of extra server space is being taken up with these duplicates. I would really appreciate any advice as to the best way to delete these duplicate copies.

My other question is about photos that link to the original full size photo. Often students would upload original images that are 3mb+ in size. These get scaled down to insert into posts but the smaller photos you can click on and they take you to a full-size original. I would like to remove the originals to make backups easier and smaller. I would just delete originals but I would much rather not have broken links. Ideally I would just like the photos not to link to one that is the larger size. Again, any ideas for a solution to this would be much appreciated.

Thanks in advance for any advice!