I'm PC based, so it might be a little different for you, but first, make sure your options are set to show file names instead of captions for media. For me to do that it's tools>options>general>uncheck box to use captions instead of file names. In the media workspace, choose to list your media by source. Since the file names are sequences (ex: 1880 United States Fedearl Census-15.jpg) you are looking for gaps in the numbering. Look at the images for each source group and write down the numbers for any gaps in the numbering sequence, and make a note of what the last numbered file is (because you'll want to delete all higher numbered files from your folder). Then just delete the files in the media folder that correspond to your list.

Does that make sense? It is time consuming, but it didn't take me nearly as long to do as I thought it would. I sent screen shots to support and got a note back saying they were working on the issue - that was early March, I believe.