Well, if you mean you have a image on your PC, you'll want to upload it to a website such as,
[url]imgsafe.org[/url]
[url]imgur.com[/url]
If you have a image online, hover your mouse over the image and right click. Select " Copy Image Address "
Once you have done that, go to your profile and input the URL.
Sig's is done the same way.