Step 1: Click on Prepare for submission You can see four tabs which are asking for screenshots

1.iPhone 6.5” Display (Optional)

2.iPhone 5.5” Display

3.iPad Pro (3rd Gen) 12.9” Display(Optional)

4.iPad Pro (2nd Gen) 12.9” Display

Step 2: Drag and drop the respective image or click on Choose files Minimum 5 screenshots need to be added for iPhone 5.5” and iPad Pro 12.9” Display to publish the app in AppStore NOTE: “In view all sizes in media manager” there is an option to upload the screenshots for all screen sizes but as you can see in the below images the screenshots for the other dimension mobile phones like 4.7”, 4” and 3.5” the 5.5” inch display will be used as default and for 11”,10.5” & 9.7” the 12.9” inch display will be used as default (So there is no requirement to upload the screenshots for each dimension)