Pixelmator is a solid choice. For years, I ran Photoshop Elements 4 on my G4 iBook as my 'on the road' image editor, and it was a fairly comprehensive suite of editing tools. You can find it pretty easily on eBay. If your Mom is accustomed to rockin' out on Photoshop CS5, then Elements 4 might seem a little puny. However, if she's looking for a decent, easy to use editor, Elements 4 isn't bad. Plus, it's not always a bad idea to run software that is contemporary with the machine you're using.
You might also want to look at Acorn 1.5.5, which is designed for Macs running 10.4 and later. You can find it at flyingmeat.com/acorn/ Which version of OS X does the Mini have?
As an aside, Preview, which has shipped with every Mac since forever, has a decent set of editing tools. Might be worth seeing which version of OS X your Mom is running (Preview is slightly different based on OS release).