upload picture you want to pixlr:
use the top left tool, the one with the two black corners to crop the image (when you only want a part or closeup of the original image)
if you use it, select the area you want and double click in the grid to save
now you probably wont have a 50x50 pixel image so click "image" in the heading
then "image size" (uncheck constrain proportions) and set size to 50pixel x 50pixel
then save the file to your computer
then upload the file as your icon and boom you're done !