Why are my texts being received as picture message emails?

When I send a text from work to my wife from my iphone 4 she receives an email on her iphone 3GS and her ipad 2 from <my phone number>@media messaging.o2.co.uk. The email says 'Here's a picture message you've been sent from <my phone number>. The message has no other content even though it is indicated that there is an attachment.

Is this a problem with my network provider or are there some settings which need changing?