I tried it on one of my random pics to see if I could get it to work, what do you think? Its a few steps instead of just one picture to import but its easy. I bet you could touch it up a bit too.
I saved and opened up your avatar in gimp then used the select by color tool (has three blocks in it that are red blue green) then select the lines that make up the image. Once everything is highlighted use the paint bucket tool and pick a lighter grayish color. Once thats done open up your picture in another box then add a new layer then cut copy paste your new grey avatar onto your bud shot. It the only way I could think of doing it.