This will increase your chance of getting back a lost iPhone. There are a few decent people in the world, that would love to return something to it's rightful owner when they find it lying around. But how will they contact you?
Step 1: Retreive an Image You Like
There are a robillion sources for pictures, photos, and artwork.
I'm going to demonstrate using a default iPhone wallpaper to accomplish the task.
I just grabbed a few I liked from idownloadblog.com/2013/09/11/new-ios-7-wallpapers
Step 2: Resize to Fit Your IPhone Model
Since I'm stuck with the old iPhone 4 at the moment, I needed to re-size the artwork (which was designed for the 5 I believe).
Find out the resolution of your iPhone easily at the beautifully simple iphoneresolution.com
I re-sized the picture to the correct resolution to my model using Pixlr, my favourite photo-editing tool. Click here for the Pixlr Editor. It's as easy as hitting "Image" > "Image Size" and picking a new size. In some cases you may want to use the Crop Tool so you don't distort the image you are trying to use.
Step 3: Decorate the Picture and Add Contact Info
Using your favourite tool, add your own flair and some contact information.
I don't recommend putting your cell phone number on it seeing as it would just ring the device the person is currently holding. I put my email address so that if a benevolent soul picks up my lost iPhone, they can contact me.
Step 4: Upload Picture to Your IPhone
Once you have a nice PNG file, use iTunes to sync the pictures to your iPhone. You can use other things like Google Drive or whatever it is that floats your boat.
Apply it in your iPhone's Wallpaper settings and you're done!