You need to find the picture on the internet and post the URL (the URL being the code-like thing at the top of your browser e.g. "en.ratchet-galaxy.com/" is this website's URL) so anyway, copy and paste the URL of a picture into the text box and it should work.
Unfortunately, you can only use online pictures and not pictures saved to your computer's hard disc.
This is the Signature Box. Writing in it means that things will appear under your posts; write as you please, for example: Why not describe the purpose of the Signature Box?