Artisan - if you are trying to upload your pic from your computer to the site directly, there are some requirements for the pic - file size and physical size. If your image is too big in either of these ways, you can resize it, first (if necessary) physically (which will also reduce the filesize at the same time) - then, if the filesize is still too big, try reducing the quality of the image. I recommend saving the image file with a new name each time, to CYA.
If you are trying to use a hosted image which resides on the web somewhere, there are also the same image size requirements - although the values might be a bit more forgiving for a web-based image used as an avatar here.