That's an MMS message. I didn't see any MMS commands in the SIM900 AT Commands manual, just in the SIM800 AT Commands manual. Which chip is on your GSM Shield?
Update: Here is the basic AT Command Manual for the SIM900:
https://simcom.ee/documents/SIM900/SIM900_AT%20Command%20Manual_V1.11.pdf
Note: Their HTTPS certificate is out of date so you may have some problems viewing and/or downloading the document.