Differential compression software?

Hello,

I'm looking for a software alternative to the commercial https://www.smartversion.com/smartversion.htm .
My use case:
I have a lot of disk images (.iso/.img, .etc) nearly identical but with small changes here and there. What I'd like is to keep only one image as a reference, and create differential archives that can be used to construct the other images. Not sure if this makes sense. SmartVersion is very good but for my purposes i need a free software.

Thank you.

xdelta.