Yeah, I would be very suspicious that tbe Gimp issue is X11 related, and the answer is probably just avoid Gimp. Which I do generally anyway, as apart from price Gimp seems to make its goal in life to combine both the flaws of poorly designs open source software, and all the flaws of photoshop.

Both Pixelmator and Acorn are good mini-photoshop style apps, both seem to work fine for most things (I mostly use Pixelmator when I need a photoshop substitute, though I virtuously seem to have actual licences for both via various macupdate deals. Acorn has the nice feature that you can write plugins in Python, but I've yet to find a reason to do so.). I also recommend taking a look at Artrage 2, which is commercial 'natural media' style graphics program, very cheap and also has a genuine usable free version.