Interesting if the size of the image is the issue.
I'm having the same lockup problem if I try to paste a PrtSc (screenshot) into Paint.NET 3.5.
My screen size is 1680 x 1050 with 32bpp.
Interesting thing, though...
EDIT: If I press PrtSc and go into MSPaint and paste, select all, copy, then go to Paint.NET, the paste works fine. So, I don't think it is related to size at all.