I know this is an old thread, but I have finally figured out how to do this!
You have to locate where Outlook places the files for your signatures. Here are the locations on the hard drive for Vista/Windows 7 machines:
Windows 7 and Windows Vista
(You'll need to be able to view Hidden Files to see the AppData folder)
Once you get there, open the HTML file for the signature that you want to open by right clicking on the file, and selecting 'Open With' Microsoft Word.
Once open, double click on the jpg you have in your signature block. Then select 'Wrap Text' in the header and select 'Square'. Close and save the document, and that's it!
Hope this helps.