Outlook web access does not allow the inclusion of images by default. However it is possible to embed the image within the signature.
First upload the image you wish to include to a web server and make a note of the full path. ie, http://www.yourdomain.com/images/companylogo.jpg.
Then create a new signature in Outlook and ensure the path of the image on the signature points to your webserver. You can find the raw signature files in “C:documents and settingsusername.domainapplication datamicrosoftsignatures” on Office 2007/XP.
You can then edit the raw signature in Notepad for example. This is an ideal opportunity to tidy up the messy html created by Outlook when designing the signature in the first place. Find the image src and edit to point to the full path of the hosted image
Send an email with the signature embedded to the users email address and open the email within OWA (in IE). Copy the signature then go into Options -> Email Signatures and paste in the signature.