Below are  a few examples of 512 X 512 (pixels) .JPG files.  Yours should be the EXACT same dimension/file type.

TIP: if your App is featured on the App Store, Apple will use this image

TIP: Please note that there are .JPG files not .PNG files

THESE FILES ARE BIG - it might take a minute to load in your browser.  Be patient.

Example 1:

Example 2:

Example 3:

Example 4: