I've just downloaded it (yesterday) and already it won't let me copy and paste. I'm using the polygon select and I can't get it to copy and paste into another picture.
GraphicConverter has shipped with an awful lot of Macs. I was a paid user long before then. This is the first I have ever heard of problems with copy & paste. After you make your selection, what are you doing to copy it?
I would go to Edit-Copy, then Edit Paste, but it works with the lasso tool. Even using ctl-C and ctl-V. I think the problem is the polygon selection won't close or connect to the first point, therefore not letting me select it. And I can't get it to do so.