How can I avoid being charged on my iPhone, for sending multimedia messages, thru imessage?
If you are sending iMessages (that contain photos or other attachments that would normally be classed as a MMS message when sending a none iMessage) then you shouldnt be charged a MMS fee by O2, you would use data while sending these iMessages, but providing you are within your data allowance, then you should'nt be charged.
iMessages are sent through Apple's servers/system (and for want of a better word) bypass the MMS fee that a mobile network can charge for MMS messages.
If you are sending a message to a none iPhone user (the name of the receipent will be green, rather than the blue it would be for an iMessage) then you can incur a charge for sending messages that have attachments (such as photos, video etc).
If you don't want to totally turn off the ability to send MMS messages, then its very easy to accidentally insert text in the 'SUBJECT' field in the messaging app. Once you insert text into the Subject field, the message then turns from a standard text message into a MMS message (this is the case even if you don't insert an attachment such as photo, video) entering text into the Subject field alone turns the message into a MMS message.
I made a video guide a while back showing how you can avoid doing this and showing how to turn off the Subject field. I must admit to entering text myself into the Subject field and incurring MMS charges, its very easily done
The video guide is for iOS 6, but it still should be pretty much the same procedure for later versions of iOS.