Well if you have a wireless HP printer then HP do their own app to print photos.
Theres also one called iprintapp that does the same for other makes - as long as its a postscript printer it should find it automatically.
Both these only print photos wirelessly - but obv in actuality you could print anything as long as you can display it on the screen as you can do a screen capture to the camera roll. Iprintapp also has a benefit over the HP version in that you can resize pics to fill half page/full page.