Pload managed to do this for his prison escape. The picture filled the whole screen.
Commandtoclient centerprint and I Think you use <bitmap:blah>.
I was going to use CenterPrint but then I ran into the height limit.
Maybe you could register a GUI with RTB and then display the image? Not sure if RTB allows images.
IIRC AFAIK he used a StaticShape which doesn't work in my case since it's visible to everyone else.