Not too sure if you need the info to create the signature as well as add the pic, but I'll chuck it in there anyway, just in case.
To add a sig:
Click My Account in top right hand corner of HC page
Locate Signature section at bottom of My Account Page
Enter signature in Enter your personal public signature box. Note: there are restrictions to size that you can have... not too sure what they are, but keep it small.
To Add a pic to sig:
Place pic on web in public accessible location (some people use photobucket, or some other free sites, or you can use your own website or whatever)
In the Enter your personal public signature box, type the html img tag followed by the URL for the picture bracketed by < and >.
e.g.
< img src=http://mysite.com.au/mypics/mypic.gif >
Note: do not put spaces after the opening < and before the closing > as I have above, I have included them there so you can see the html code, otherwise this msg would have a broken link and I would not be able to display the example.
eg:
That is the same html code as above without the spaces, and your browser is trying to find a picture located at that http://mysite.com.au/mypics/mypic.gif url...
Note: That's how one posts charts and stuff in msgs.