Welcome to O2.
Please go to http://mmsi2legacy.o2.co.uk to retrieve your MMS.
Below is the quoted message from the website:
View your media message
If you have been sent a text advising you have received a picture message, you can view it here. Simply enter you mobile number and the pin you received in the text message.
NOTE: O2UK will store your message for SEVEN (7) days from the day you receive the text notification.
Please enter your ID exactly as it looks in the text notification you received e.g. 447712345678.
This is a 6 digit number that you received in the text informing you that a Picture Message had been sent to your account. Please enter the full pin without any spaces or non-numeric characters.