When you add screenshot to your app you are going to publish, you will see this “165 x 145 px, optional”.

The 165 x 145 px, is to small. It is just like a 2 x 2 ID picture and useless to present the highlight of the app pages. 

Putting optional mislead developers, other had attached images bigger than the snapshot viewer and still useless to present the highlight of the app pages. 

I recommend changing it to 640 x 480 pixels, it is equivalent to web image-large, enough to fit the snapshot viewer...