Hi Anton!  Ok, here's how I did this pic, and mind you this was thrown together while I was sitting on SL and a VERY fast job.

First, I took the pic of the avatar bunny, saved to disk.  I also had a photo of myself I took with a digital camera handy as well.

Second, opened up "Gimp", which is photoshop for those of us who can't afford photoshop.  Opened up the SL image, and my image.

Third, cropped out some of the background of the SL image, just to center subject and to accomidate for my RL self.

Fourth, and this was the hard part.  I took the pic of myself and did work on it.  I deleted the background (which was white, thankfully), and used the "lasso" tool to cut that image out, and copy into RAM.

Fifth, pasted the RL image of me into the SL backdrop as a new layer.  Layering is a way to work on multiple parts of an image, but other people can explain it MUCH better than I can.  Once the new layer was set in place, I used the eraser tool to remove some of my snowboard so it would look like it was behind my avatar's elbow.

Then, saved and posted to koinup.  :)

Now if I wanted to take some time, I could adjust the layers to merge the images better, or feather the edges of myself to blend it into the S L world.  As it was the only thing I played with was I pixelated myself JUST slightly, which is a filter in Gimp.