No extra cost, it's all part of your texting. Just put the email address in the TO: box instead of a phone number. You can actually send pictures to Facebook three different ways:
I posted 3 pics (successfully!) to my Facebook account.
I sent the first to 32665, and I put different text in the Subject and text fields of the message. Both showed up in the caption, Subject text as one line, Text as the second line.
I sent the second to firstname.lastname@example.org - same results.
I sent the third to my personalized email@example.com - same results.
On my FB page, they showed up as one post, 3 photos. (I sent them within seconds of each other, so I suppose that's why they all showed up together.)
It looks like they all work, so use whichever is easiest for you! I have all three listed under my Facebook contact, but it doesn't seem to matter which is used, they all produced the same results.
You can also send pics to ANY email address - I send to my sister who as an AT&T phone but no picture messaging.