Profile picture or avatar? You can use an image viewing program to resize or crop the picture. I use
FastStone Image Viewer for all of my image viewing and minor editing.
I've also noticed it's much easier to use an image you have saved on your computer, rather than link the image you want to use to a web address.
Note: For Profile Pictures the maximum size of your custom image is 600 by 400 pixels or 128.0 KB (whichever is smaller).
Note: For Avatars the maximum size of your custom image is 160 by 120 pixels or 19.5 KB (whichever is smaller).