Mrsberry does some great custom free themes and recently has been posting some nice wallpapers. The latest wallpapers are called Girl Silhouette which features a girl standing next to a Blackberry logo. The wallpapers come in 13 different variations in a 480×350 Px. You can check out the full gallery at this link.
Some of the other free wallpapers include a Blackberry logo with a fluorescent aura around the logo.
To check out/download this free wallpaper head over to mrsberry.de she has released several free themes that will catch your attention as well.