Hm.. interesting. I cannot reproduce this one. What browser/OS do you use? Does it happen to other head types? Does changing colors affect it in any way?
As for why: I use a couple of colors as key colors in the templates to replace them with whatever the user chooses. In particular for skin it's green (00ff00) and dark green (008800) and outline is yellow (ffff00). The colors on the image you've send are a bit off (07fe00, 008b00, f7fc00) so they weren't replaced; but why only on the edges?
There is also a 5% shadow to the left of the head (not visible really, but you'll find it when you inspect the file) this looks like a clue. I'm especially puzzled that it on'y happens to two frames and only part of the picture; super weird. I'll be looking into that.