Found this on epic forums...

Bindings=(Name="F8",Command="togglescreenshotmode" ,Control=False,Shift=False,Alt=False)
Bindings=(Name="F9",Command="tiledshot",Control=False,Shift=False,Alt=False)

By default F8 and F9 are open keys (at least on my game) so just copy and paste into the UTInput file.

While spectating hit F8 first to enter screenshot mode. Then F9 to take the hi res shot. It basically takes like 6 screenshot of differnt parts of your screen then generates a nice pic from all the shots. Mine was putting out like 20mb pictures. Have one set now as a bg on my desktop. try it out!