I got an App called Ensoul some time ago. This application is designed by MacPaw, a Ukraine-based studio. This app helps you with rather simple task – to make wallpaper picture fit exactly the dimensions of iPhone screen. Of course you can do it right in the Settings app on iPhone, but the problem is that you can't adjust the picture size and position it on the screen with pixel precision. Another thing Ensoul offers you is to assign a portrait to each of your contacts, allowing to resize the pics in the same way.
Guys at MacPaw launched a special site Ensoul.me where you may share your pictures or get awesome ones from community. I've also selected 14 photographs as iPhone wallpapers. You will find them in Ensoul or on this page below. After clicking the picture you'll continue to a page with a wallpaper preview on iPhone and a download link.
Enjoy! And don't hesitate to click :)