I don't think Rivals hosts images so just upload the pic somewhere, copy the link and paste it back here. You'll probably need to format it with img and /img tags (in brackets) before and after the link, although some sites will do them for you.
[ img ]hostsite.com/image.jpg[ /img ] no spaces in the brackets
Or hit the little square with the tiny mountains in the reply box and copy the link there, does the same thing essentially. Tomato, tomato.