Best Free (or non subscription)editing program for stitching scanned artwork

HELP!

I used photoshop many, many years ago, but they've gone to month to month subscription, and I don't need it often enough to warrant that. My non-tech savvy aunt is wanting to digitize her portfolio of paintings and she is wanting to know how to do the scanning and stitching on her own. I am at a loss for what to recommend for high resolution photo stitching. She doesn't need loads of creative options.

Any inexpensive options out there for her? Thanks!