I guess you are kinda right tariq...well here is way that i use to put image planes into layers...so that i can manipulate them...so here is my way. let's say i want to put a profile pic in the side view...what i will do is :
in my side viewport... View>Image Plane >Import image...
i will select the image i wana put in that viewport.
after setting the scale or other properties...
i will create a new layer. and in my Perspective Viewport i will drag a selection around the image plane. and image plane will be selected...now you can assign it to the layer....
works fine for me....
p.s: one it gave me an error and did'nt let me select the image plane while dragging a selection around it.... so i just closed and restarted maya... dunno what kinda prob was that though.,...
so Black Robe, you check it out dude....it will work