Look at this post.
It worked in Excel 2003, but I haven't been able to test it with Excel 2007.
The link in the post does not work, but there is a description of the registry entry to create.
Backup your registry before attempting to make any changes to it and create a Restore point. Incorrect registry changes may make your PC unstartable.
If your not comfortable with changing the Registry it might be better to live with your browser opening jpegs.