Third party functionality keeps adding more and more features to the iPhone. HP has just introduced (Store Link) a new app that lets you print your pictures to an HP printer.
From HP:
HP iPrint Photo is a free downloadable software application that allows you to quickly and easily print 4" x 6" photos. Images stored on your iPhone or iPod touch can print on most HP networked ink jet printers connected to a local WiFi network. Wireless printing is a fun extension of the iPhone / iP
