I need an app where I can make a collage?

I have an iPhone and I need an app where I can make a collage with a lot of pictures… Like 20.
Also, does anyone know how to make it so that when I set a photo to be my wallpaper it doesn't zoom in?

Photo Grid. For the wallpaper situation, if it zooms in, try using Squaready. It's also perfect for posting pictures on Instagram that you don't want to crop.

Picstitch