Hi JDS,
I've always put the snapshot builds and the bi-weekly betas in my Forge folder. Just unzip the archive to that directory and allow it to replace files.
Additionally, I always run the Content Downloader with each new Beta. I choose the first option in the list "Download Default Card Image."
Of the four Magic AI games on here that I play, I find updating Forge to be easiest, followed next by Magarena, BotArena (though BotArena has the longest time interval between updates so I have to go back and review

), then finally Manalink 3.0.
One last note: if you aren't getting LQ pics on the Content Downloader that is usually because they aren't available yet. The developers of Forge do not control the availability of LQ pics. That said, if you look around in both the Forge thread itself and the Card Pics thread for all programs, you will usually find LQ pics and at least for me, I have found them already renamed for Forge and it's just a process of downloading these images and placing them in the proper directory. I've done this for Khans of Tarkir, Journey into Nyx, and Magic 2015.
If you need help as to where images go, just let me know.
Keep Tapping!
Vlad
PS: Forge is easy to update because you don't need the previous builds. Just grab the latest successful snapshot and extract as mentioned. Manalink 3.0 on the other hand does require each incremental update because they have revamped and broadened the scope - considerably - of MicroProse's classic Magic The Gathering: Duels of the Planeswalkers game.