You'll have to create the folder LFGFRAME inside your Interface folder. Then put in your remade version of
UI-LFG-ICON-PORTRAITROLES.tga. The image has to be in the same size and layout as the one I linked (leader-healer-tank-deeps). That should replace the icons on the party frames. You will have to exit completely out of the game before it will read the new file though.