You have two choices, make one yourself or grab one off the internet.
Personally I took some nice photos and used one that's pretty cool, that's been my lock screen for years.
A company logo can work well, as can album cover or movie art, whatever your into.
A cool idea is to do an image search for optical illusions and use one of them. It's also a cool way to know which contact is calling if they didn't post a nice pic on Facebook