If your sure you have credit on your phone as they cost 16p follow this guide
I have plenty of credit so that isn't a problem.
I have already done everything that your link says to do so I guess that leaves me to try downloading a Giffgaff APN from Android Market - have you any handy links to get me to the right place please????
MMS cost 16p each to send. You need credit as they are excluded from all goodybags.
Double check settings too.
As you are using an Android phone, get giffgaffAPN App from Andorid Market. (Connect via wi-fi to download it).
Run giffgaffAPN, delete ALL traces of previous APN's, then run it again and install full giffgaff web and MMS settings with a couple of button presses. This should fix the problem. Works fine on 2.3.4