Hello,
I can write a script for you that would read the contents of that folder "propertyimages" and the file names from the database, and then build a list of files which are no longer referenced in the database.
Then, you can use that list to move the extra images to another folder.
Also, if you wish to reduce overall file sizes, I could download the images to my computer and resize any image larger than some resolution (for example anything higher than 2000 x 2000 pixels) to something reasonable like 1600 x something, preserving aspect ratio.
Note though that you seem to have 531k photos that use 36 GB of disk space, so an average of around 70 KB per picture, which seems reasonable. There may not be anything you can really do about it, or the savings would be small.
I would probably need access to that control panel, user and password would be fastest, but if you don't want to give that, or maybe you can give me a read only ftp access to that propertyimages folder and an export of your database. Working through Teamviewer would also be possible (as long as you have a decent internet connection)