I think they are birch trees (Betula), based on the white bark with black horizontal patterns (Lenticels). The bark patterns at the base of the trees are also very similar to birch. Either way (relating to one of the comments), it's an autumn picture, where both birch and aspen have yellow leafs (aspen leaves can also be red or reddish, especially at the end of branches).
This page also agrees with me (which doesn't necessarily have to mean that much though), which labels the exact same picture as "birch-tree-wallpaper-3.jpg".
