Some time ago, I created a tumblr account (which can be found here). I'm not sure if I will ever get around to posting anything in it (or figuring out how to use it), but if anyone wants to make requests there, feel free to do so.
There may be others not currently listed. You can search the reference gallery if you can't find what you're looking for here. Please note that if you can't find something after searching for it, you can always make a request.
This is utterly amazing, and I wish to beg you for shots of the Mass Effect 3 Geth Prime (from the Multiplayer).
And on a related note, I am desperately trying to find references for the Biotic Hammer/Melee that the Krogan Sentinel uses in ME3 Multiplayer, but I can't find it anywhere! Including poking around the PCC files. If its not a bother for you, OR if you could give me pointers, I would deeply appreciate it!