1) get someone to host the picture (could be you, or you could E-mail Ziroc. You can also take it directly from the site where you found it if you E-mail the webmaster for permission.
2) put the link in your sig (edit your profile), like (in my case)
http://www.angelfire.com/realm/spijl/wc3ranger.gif
3) put the link between IMG tags, like: [ IMG ]
http://www.angelfire.com/realm/spijl/wc3ranger.gif [ /IMG ]
but without the spaces (both between the brackets and the IMG and between the IMG and the address
4) save your profile and you're good to go