Weird, i've never known the image plane to loop. Generally by default when you enable the image plane, it puts a expression in the field to make the image plane frame # sync up with the frame number on the time slider. So it should never loop. If you've manually set keys (by breaking the expression), it still shouldn't loop, unless you've selected the curve in the graph editor and told it to. I guess that would be the first thing i would check, is the graph editor and see if you've got your curve set to loop in post-infinity.