Do I just download those pics and put them into the skins folder to use them?
Runs flawlessly on 5S so far.
If you click the links, they'll take you to
GBA4iOSkins.com. You can pick through them there, and either download through your computer, or just do it straight from the device. Clicking download on your phone will prompt you to "Open in GBA4iOS?". They'll show up in the skin section after that.
Does this work with the official iOS7 controllers? (and by extension the ControllersForAll tweak that allows PS3 or PS4 controllers to be used?)
Yes for iOS 7 controllers. As for ControllersForAll, I definitely remember reading something about it working, but can't verify 100%. I
think it does.
I think I remember the GBC having a function where you could pick a color pallet when playing regular GB games. You'd have to hold down a button and a direction when starting up the system. Is that ability on here?
You're remembering correctly. I don't think that works with this though, since you don't have access to the controls before the title screen is present. I tried with Donkey Kong '94, and couldn't get it to work anyway.