The Street Artist app for iOS lets you use your iPhone, iPod touch or iPad to superimpose images over the world around you, and snap photos to create street art murals that you can share with friends.
-- Select artwork from the Pasteboard, from the Camera Roll, or from the app's shared Documents area (shared from iTunes).
-- Adjust the transparency of the artwork to allow the textures of buildings and the street and anything else to show through.
-- Directly share your street art using email, Facebook, Twitter, Flickr, or to apps like Instagram.
When finding art to use in the app, do one or more of the following:
(a) Open Safari and search the web for something like "graffiti png". Best are png type images, with transparent backgrounds. Tap and hold on an image, and "copy" to your device's pasteboard.
(b) Same as (a) above, but tap and hold on an image and "save image" to save it to your camera roll.
(c) Gather some png images on your computer, open iTunes, select your device, select the "Apps" tab, find "Street Artist" in the "File Sharing" list, and drag your png images into the app's "Documents" area.
All three of these will make the image selectable inside the app.
NOTE: You may also want to use an image editing app to erase some of the image to create a transparent background. These apps typically let you copy the edited image to your Pasteboard or to your Camera Roll... where they will then be accessible to the Street Artist app.