#1: Find your SWTOR folder.
#2 Go into: swtor -> retailclient -> LoadingScreens
#3 Rename your chosen picture into "LoadingScreen", and replace it with the one already there.
I advice you to save the other one, because you will have to have the original one before every patch, apparantly. Or else the gamefiles will corrupt.
Anyways, here's mine. Until i decide to change it...
http://i44.tinypic.com/33dau50.jpg