If someone attempts to send your Google voice number an MMS you will receive a notification about the message and the image will be sent to your email address.
http://googlevoiceblog.blogspot.com/2011/10/first-steps-towa...
http://googlevoiceblog.blogspot.com/2011/10/first-steps-towa...
When I get MMS from AT&T or Verizon, then I don't get anything but from Sprint customers, I get the notification and its in my gmail account right away. Probably due to the integration with Sprint and Google Voice. http://www.google.com/googlevoice/sprint/.