a) If you don't have a 500 x 500 pixel image, resize your photo first using image resizing software like Adobe Photoshop or Pixlr Editor. If the image is bigger or smaller 500 by 500 pixels, it might come out weird!
b) Upload your chosen photo to your Facebook wall, Tumblr, wherever you're comfortable posting it to. If you're concerned about your privacy, make sure you upload it somewhere you can easily delete it!
The link should look something like this: