I really love the cage transform tool in the GIMP. It was first developed by Google Summer of Code student Michael Muré in 2010 and finished by Gimp developer Alexia Death. It allows you to define an area within an image (in my case, the four corners of the whiteboard frame) and drag on those points to stretch the image out. For this whiteboard photo that was taken at an angle, this process resulted in a straightened-out image of the whiteboard. (I followed up with a Difference of Gaussians cleanup that Garrett taught me a while back 🙂 ) It’s a pretty magical tool. Give it a try!