1. To answer your first question, for signature you need to use images tags like these:
E.g.
Would be the top choice tags on
Filehive, and would look like this:
[IMG]
http://www.filehive.com/files/1123/P...c-DMC-FZ20.jpg [/IMG]
Except no spaces at all, I had to leave spaces so it displayed the text, you only need to copy and paste the link from the image uploader anyway.
2. Secondly, you will find a guide on changing a phpBB board header here:
http://forum.forumer.com/showthread.php?t=13042