Sorry to hear you've been unexpectedly charged for sending picture messages.
I can see badoit has made a point around emojis already. This is more thank likely the cause of these charges. To help shed further light on this for you here is a link to a previous thread which explains a bit more around this situation for you.
Further to the other replies, sending really long messages and/or having a large circulation list sometimes results in SMS being converted to MMS. I don't know if you're charged 40p for each recipient.
It's the handset's doing. The UK is still pretty backward in that MMS are charged differently to SMS. In other countries, carriers treat them the same as far as charging goes, which is why the handset converts messages if MMS transfer would be more efficient.