What phone are you using please? You need to check that you have all the correct network settings in your phone.
giffgaff SIM cards do not always automatically put the required settings into the phone for you.
You also need to have credit on your GG account in order to send picture messages. MMS are NOT included in any goodybag.
So if you topped up with £10 then converted that £10 into a £10 goodybag, you have £0 credit therefore you cannot send MMS.