On your HTC Desire S (possibly other Android Phones) go to > Settings > wireless and network > mobile networks > Access Point Names
then hit the menu button and add a new APN. Don't forget to save once you have added the detail ;-)
Name: O2 PrePay Web/MMS
APN: payandgo.o2.co.uk
Proxy: leave empty
Port:
Username: payandgo
Password: password
Server:
MMSC: http://mmsc.mms.o2.co.uk:8002
MMS Proxy: 82.132.254.1
MMS port: 8080
MCC: 234
MNC: 10
Auth Type:
APN protocol: IPv4
Don't forget to hit the menu button and save!!
Once you have done this you can then MMS pictures to twitter. Well you can if you are an O2 customer! ;-)
Have fun.
Cheers
m