You need data enabled to send/receive pictures on the HSPA network. Note that your plan includes unlimited MMS but if you do not have a data plan, then things may update in the background like FB, emails, weather, etc. and you'll get charged pay-per-use data.
A way around this is to look at alternative methods of sending the picture over the internet through WiFi, such as BBM, email, FB, WhatsApp, etc.
Or... you can edit the APNs in the phone to allow pic messages but not allow data. Instructions here: