No such tendency here:
Jan: 0,48
Feb: 0,54
Mar: 0,50
Apr: 0,52
May: 0,48
Jun: 0,56
Jul: 0,56
Aug: 0,50
Sep: 0,50
Overall average: 0,52
I think maybe the type of pictures you sell will have something to say. For instance, while textures aren't the hottest subject around, I believe they're fairly more used by print designers rather than web site owners, so they'll want them in larger sizes. Other shots on the other hand, will be more used on web sites.
So, maybe the pics you have uploaded in the beginning of the year were more used in print.