Sending a text of any sort (photo or text only) to an email address is an MMS. Since you have unlimited texts you will not be charged for MMS.
However, data will need to be enabled to do this. The data usage required will not count as chargable data so it will still be free, but anything your phone does otherwise on cellular data (updates to applications, etc.) will count toward monthly usage.
Since you didn't mention it when you described your plan I'm guessing you don't have a data add-on, and the new plans with the flexible data added only go as low as $20 so your $15 plan doesn't include data. Therefore, any data you use will fall into pay-per-use which is $3/MB for the first five MBs and the $0.05/MB after that. (Ouch!)
I would recommend setting up a data plan if you're looking to activate data for any reason.
(Of course the above warning can be mostly ignored if you don't currently own a smartphone. What phone are you using?)