I was just looking at the settings the other day. Per other comments in this topic, I'd look at disabling the MMS features; IIRC TextSecure also has user settings for this.

Edit: Just had a look. I do not have TextSecure as my default client. There is MMS configuration information, but not a simple "disable automatic retrieval" or similar setting, as there is in my default SMS/MMS client. I don't know whether one appears when TextSecure is the default client; I suspect not, and maybe this should be addressed?