SearchMyFiles  = You could use this to search, but mainly it's a good free duplicate file finder.
Everything  = This tool is VERY fast for doing a rapid initial searches so you can identify the locations of images (again, you need to specify the extensions). You can also limit the size so you don't end up hitting thousands of tiny GIFs. Unsure what versions of Windows this supports, but SearchMyFiles probably supports more older systems.
Bulk Rename Utility  = Fantastic renaming utility which you can use to add prefixes or whatever to file sets en masse, which will help you or anyone else later search the resulting files you find, not to mention keep things organized and avoiding duplicate file names. It also will extract the "Taken" date from an image (if it is embedded) so you can use that in the file name automatically.
IrfanView  = This free tool has a rapid thumbnail viewer and batch converter that is very handy, but remember to check the option to retain the original file date stamp if you're converting anything for any reason. Date stamps on images are very important.
Don't forget movie files often stored with images (.MOV, .MPG, .QT, etc). These can eat a lot of space. I used HandBrake to convert a lot of large uncompressed .MOV files to MP4. Of course with storage so inexpensive these days, I would keep the uncompressed movies for the sake of preservation.
In your project I would likely have top-level folders for each person's collected photos or device, then sort the images/movies within by year folders.