Marvin, good afternoon. To move the image while making a selection, keep
pressing down on the mouse button while simultaneously pressing and holding
the space bar. You'll get a hand icon at that point, and then movements of
the mouse will move the image in the window. Return to where you left off
on the selection (all the time holding down the mouse button) and when you
get to that point, release the space bar but not the mouse button and resume
your selection. Takes a little practice, I've found, but it works
eventually.

Hope that helps!

Chuck