Outlook doesn't handle tables in signature files well at all. If you get rid of the table in that code and put the text below your graphic, it will work. If you get rid of the graphic completely and eliminate the table, it will work. I tried three different iterations of it with a table and was able to reproduce your error in OLK 2003 and 2007 every time. Get rid of the table and I can no longer reproduce the error. It looks like a limitation within Outlook, though I cannot find any documentation anywhere to show that.
Justin