Remember "Only one image can be displayed at a time, its width can be no greater than 150 pixels, the height no greater than 150 pixels, and the file size no more than 24 KB. "
Make sure the file you're trying to use meets those rules. You can click the right mouse button on it, and select properties, and it should tell you the filesize and dimensions of the picture.
Then click here
to edit your profile where you can upload your image. It's as easy as using the browse button to local the file on your computer, then click the submit button. It takes a second or two to upload, but it should be there after that.