Most of my friends have iPhones, so this happens to me a lot. Every time I'm involved in a group message with iPhone users, Apple sends it as an iMessage message. But since I'm on an Android device (Galaxy Nexus), I see it as an MMS message. However, it never ever works for me. I see it as a blank message and it tries to download it as an image.
I have my mms settings on my phone set to go through http://mms.plspictures.com per the Ting instructions. Is my issue a Ting issue? A plspictures issue? an Android issue? Or user error?