Quoted directly from Apple's In App Purchase Guidelines
For review purposes only. Your In App Purchase screenshot will not display on the App Store or on the iPhone device store. Once you have tested your In App Purchase and are ready to sign off, upload a screenshot of your In App Purchase in this section. The screenshot should be a clear picture of your In App Purchase in action and must be 320 x 480, 480 x 320, 320 x 460, or 480 x 300 pixels.
Cheers.
UPDATE
The correct size is now 640 x 920. The other sizes are no longer accepted.
This screenshot should show situation when in-app purchase will be activated, in your case it can be screen of application without 'lite version' restriction (or directly after purchase).
I provided a screenshot of the Confirm Your In-App Purchase dialog that StoreKit presents when purchasing each product, and everything was approved by Apple the first time. I didn't show screenshots of how the app changed before & after the IAP, so it doesn't seem as though this is required.
Screenshots requirements are outlined below: iOS requires at least 640
x 920 pixels.
I'm working on this right now (end of July 2020) and only exact resolution of 640x920 uploads successfully. I tried higher resolutions and 640xH(H>920) without success.
Sorry to answer such an old question, but this aspect hasn't been addressed: I have always felt the "screen shot of your in-app purchase" language was vague. I've learned in creating 400+ IAPs that it doesn't matter what the screen shot is, as long as it is the right size. I have submitted 10+ IAPs at the same time with the same screen shot that is just a random screen shot of my app that has nothing to do with the IAPs at all, and all have been approved.