There is much debate about whether godparents should be given 'thank you' presents after the christening. This is certainly not de rigueur, and should not be expected by new godparents. However, if you really feel impelled to thank them for their time and commitment, then a framed photograph of your child (or even better the godparent and the child) would be appropriate.
There is no debate, however, about the necessity of writing thank you letters for christening or baptism gifts. These should be handwritten, carefully personalised (ie the actual present should be mentioned, with a positive comment about its appropriateness) and sent within a week of the ceremony.
Godparents should, of course, receive a special thank you for the role they have played in the ceremony.