Hi markus46.
You can add a bundle of 50 messages for £5.11, just log into "My O2" and head for "add bolt-ons".
Have you not considered sending your pictures via email or an instant messaging application, MMS scales down your images considerably and in my opinion is a very overpriced archaic service throughout the industry.
Sending through one of the above methods not only keeps your pictures looking great but also can be balanced against your data allowance.