It's thoughtful to give cards that reflect the interests of the person receiving it - or, failing that, fun to send something that you particularly like. It's surprisingly difficult, though, to find any aimed specifically at gardeners,
For Christmas cards with a gardening theme, check out Thrive's website. (Thrive is a charity working in social and therapeutic horticulture.) Their "Christmas Thyme" design is exclusive to them, but I particularly liked their well dressed shed [NB 2014: link removed as no longer viable].
Perennial has a much wider range of cards, to support their work with distressed professional horticulturalists. They've got quite a mix of gardening and non-gardening subjects and, again, I fell for the shed (pictured).
You might have spotted in the past that I have a particular soft spot for the humorous cartoons of Chris Madden which you buy under licence to produce yourself (a quick glance at his terms indicate a £15 rate for personal use). His website is much easier to navigate now, and he's got a good array of Christmas gardening subjects.