I think one of the best things I like about my iPhone (well, and my old crappy Blackberry) is snapping a pic and then setting it as the background image (screen saver, wallpaper, etc.). With the iPhone camera, which is good for a phone camera, I have started to take more than just stencil photos (I usually use my camera camera for nature shots). I took the recently-posted empty F train car photo specifically to use as wallpaper on the iPhone.
Here are three more photos I took specifically to frame in a smart phone. They are all vertical. I do take horizontal shots of some things to use on my laptop for a desktop image. Please feel free to grab these images and use for backgrounds on your phone. Just drag/drop and put in your phone’s camera roll to tap/select as wallpaper. Remember that they are all protected by the Creative Commons Attribution-NonCommercial-ShareAlike agreement. You can use them but not make $$ off of them!