Below are the requirements to have great looking images on your APP.

Let's begin with this:

If you are in your app and you see an image size by the area that you are going to be placing an image in, make the image exactly that size.

If you need assistance on cropping or resizing your images you can refer to the bellow tutorials.

Resize your image using Pixlr

Cropping Images Using Pixlr 

Full-Width View:

At least 2018x1024

Cell View:

At least 1024x1024

List View:

At least 1024x1024