By placing the picture on a web server and then link to that picture instead
of embedding it in your signature.
FWIW; whether or not an embedded picture shows up as an attachment usually
depends on the receiving client and not the sending client. There are known
cases in which a virus messes things up and the embedded picture will show
up as an attachment as well. Uninstall this integration part of your virus
scanner and try again; you'd still be sufficiently protected by your
on-access scanner part of the virus scanner. For more details see;
http://www.msoutlook.info/question/20