I have no general winter, but quite a few Christmas images (mostly food), and I've noticed that they start picking up with a few ELs in July/August (probably for Christmas cards, I suppose), and the sales grow from September on, October/early November being the peak. And they sell right up till Christmas. This holds mostly for Fotolia, where a lot of those images are exclusive. I've heard many people say that Christmas images sell all the year round, but I haven't made that experience - January till July mine sit on the shelf.