Don't forget you can mirror when drawing simply by setting SetScale -1,1. You probably knew that but maybe others didn't...
also -- if you *need* an image to be stored in flipped format, I wonder if it wouldn't be faster to to a setscale -1,1, draw it, and just use a grabimage on it (instead of re-drawing it a pixel a time)
Unless you have some alpha blending going on as well, it might be faster to do it that way?