A serious suggestion:

People say that symmetry is what leads to beauty. Taking art I recall that we were supposed to look at our pictures in mirrors to make sure they were balanced.

I suggest being able to draw a line at an arbitrary point in the picture and have one side or the other mirrored along that line. Additionally, make the mirror semi-transparent (0 to 100 or whatever) so that one side of your face is superimposed on the other. Could be useful for makeup, either as a comparison or as a guide when you have done one side of your face and not the other.