You're probably using 'goodybags' (which is what GiffGaff calls their bundles). They give you a certain number of minutes (to UK numbers only), a certain amount of data and a certain number of SMS texts (to UK numbers only). What they DON'T give you is calls and texts to overseas numbers (or the use of your phone outside the EU), calls to premium rate numbers and, most importantly here, MMS texts (which are picture messages).
So you're trying to send an MMS message but without any relevant credit on your phone. See here and click 'Add Credit' to fix the problem:
https://www.giffgaff.com/support/top-up-and-balance/credits-and-goodybags-purchase