I use Paint. Just select the area you want to be the avatar, hit Crop, right-click the picture and hit "resize", change the tab from "percent" to "pixels" and set the largest number to 150 (the largest dimention the site allows).
Then save it (making sure to remember the hight and width) and upload it to the site, put in the dimentions in the little boxes down below, and his "submit".
And there you go.
