I agree. My proposal: just implement the possibility to calculate and store the compression checksums of images in some local hash file with a documented structure. I'm sure sooner or later someone will start a web service that allows uploading/sharing of these files.
Maybe you can offer your database as an additional download to play with.
About the applications: surely there is the problem of endless JPEG quality settings, but luckily nearly all dominant image processing applications offer scripting/commandline interfaces. This surely boosts the checksum retrieval process. IrfanView for example allows the command-line conversion to JPEG and has an explicit Quality parameter.- Franz