Freebbble!

Direct download links to the Finn version. Open the Dribbble page to see the Jake and BMO wallpaper versions.

Adventure Time iPhone Wallpapers is a free Wallpaper design resource made by Dribbble user Samuel Suarez. As far as I can tell, its license is Unknown.

Helpful hint: Dribbble users are nice. If the license is unknown, it is usually because they forgot to mention it explicitly; not because they were being stingy. Ask nicely and they will probably allow you to use it.