Yesterday, when my friend droped by my place, I selected some wallpapers from interfacelift.com for him. Three of them that have lake and tree are very beautiful. Here is preview (the left in the top row is old):

I like them very much and recommend them (especially the one in the lower left corner) to everyone. You can search their names on interfacelift.com to find right size ones.

Their names: Fronzen Tree; Sunday Morning II; Lack of Dreams; Sunset at the bay.