The repositories for Mint 13 (XFCE) have not been updated to show more recent versions of my main photo software. I've learned that PPAs for them can be had here:
GIMP is up to 2.8, Rawtherapee is up to 4.10, and gThumb is up to 3.3. If using Mint 13 be sure to get the PPA for Ubuntu 12.4 Precise. I haven't found problems with installation or running them as of yet. Hope others find this useful.
(I'm still hoping for the day that "LTS" means never having to search the internet for current versions of software that should be in the Mint repositories.)