I'll leave it to you to decide how much use an iPhone application allowing direct printing from the handset to a wireless printer is. If you have an HP printer, though, and such functionality would be of use to you then be advised, there's a free app available on the App Store enabling it.
Called HP iPrint Photo, presumably meant to sound like a description of the task it enables, the app allows easy printing of a borderless 6in x 4in (American and European standard) photo. HP boasts "improved image quality from image de-noise filtering technology," which is a definite must give the awful quality of the iPhone's camera.
Printers with separate photo trays should have these automatically selected, saving the effort of having to put photo quality paper in the main paper tray for the extra lazy among you. Although, again, the poor quality of photographs taken using the iPhone would probably be a waste of decent printing material.
Still, the app is free so HP can hardly be faulted on that front.
HP iPhoto Photo (iTunes Store).