On Special:AchievementsCustomize, you've made a slight spelling error; I've copied and pasted the paragraph with the mistake in it, and highlighted the word.
"You can use rectangular pictures, but you might find that a bit gets cropped out by the frame. If you have a graphics program, then you can crop your picture to put the important part of the image in the center. If you do no have a graphics program, then just experiment with different pictures until you find the ones that work for you! If you do not like the picture that you have chosen, click "Revert to default" to go back to the original graphic. "