Wallpaper apps are commonplace, but so many have really second-rate images. On my own iPhone I prefer to use photos I have taken, especially landscapes, but Cool Wallpapers HD (free - ad supported) has some simply beautiful images that are tempting me to add them to my library.
The photos work with all iPhones including the new 6 and 6 Plus, and new images are added every day. There are more than 8,000 in the collection now. There are 14 wallpaper categories, like nature, 3D, people, abstract and so on.
You can browse through the collections as thumbnails, enlarge them to full screen, and save them to your photo library. The app notifies you of new photos, but that's a feature you can skip if you're already drowning in notifications.
Any photo can be saved as a favorite, or shared with a friend. I was struck by the high quality of many of the images, and of the many I liked, I wouldn't be ashamed to have them as my wallpaper or lock screen.
The images are sized in the following way for different iPhones: iPhone 6: 890 × 1590 pixels, iPhone 6 Plus: 1438 × 2592 pixels, iPhone 5s/5c/5: 744 × 1392 pixels.
There's nothing fancy about the app, but the image collection is a solid one. The ad banner across the bottom is a bit distracting, but most people will be able to live with it to get the quality images. Cool Wallpapers HD requires iOS 6.1 or greater. It's free; give it a go.