Simple is best! I’ve tended to find a nice big seasonal or festive banner image works best on a webpage, for adding some festive feeling. A snowy scene, children sledging or building a snow man, Christmas lights, pine cones, fireworks, carol singers, that sort of thing etc. Text can either be ‘photoshopped in’ or if the theme supports it, you could overlay the banner with some text (if you feel text is needed).
You honestly don’t need brilliant photographic skills to capture some basic images. The one below is a quick-snap I took at a local garden centre on my iPod Touch that I’ll be using in a few websites shortly:
Neither do you need to spend a small fortune on stock images. A relatively new website called Un Splash was flagged to me by a colleague a few days ago, and this has some terrific free images, including a few festive ones: