There is a ton of wallpaper apps both on iPhone and Android. But there are only a few which really stick out and are worth downloading. Wallbase was my choice for Android but unfortunately it isn't available for iPhone. I went through a whole load of apps (seriously, I went through many) and found that CoolPapers Pro was the best.
CoolPapers Pro does however cost £1.99 ($2.99), but it has a spectacular collection of high quality wallpapers, resized for iPhone screens. You can browse through many categories, ranging from celebrities to art and design. You can sort them by most popular, newest or downloads. You can also sort them by all or HD only so you can get the best quality wallpapers if you have an iPhone 4/4S. Downloading and browsing is super fast and cuts out a lot of hassle when finding wallpapers on your computer and then transferring them over to your iPhone.