BannerPic - Instagram Banner lets you create a cool banner, but you can't do much to customize it. The app only has a few standout features and most of them are tucked behind a paywall. Still, it's not a bad way to get a cool banner if you don't have the editing chops to do it yourself.
BannerPic - Instagram Banner turns any picture into a three-tiled banner that you could use on your Instagram or Facebook profile. You can take a photo from the app, but inserting a photo you already have makes the process much faster. The app chooses a part of your photo to turn into a banner, but you can adjust it or zoom in and out to your heart's content. You cannot make three pictures into a single banner in the free version of the app, though. You can't make the banner, itself, bigger or edit the photo in the free version, either. The app also puts a watermark on your banner that would be impossible to get off.
There are plenty of hang-ups in the free version of BannerPic - Instagram Banner, but the premium version doesn't seem that much better. It will give you a good-looking banner, but you'll be hard pressed to find any fun way to edit your banner from this app. At that point, you might be better off using an editor with more controls and features.