After much web searching, I couldn't find a good way to get screenshots for the Apple App Store without actually buying each and every IOS device or manipulating a lower resolution screenshot to fill in for a higher resolution IOS device. The Xcode device simulators are tempting, but you can't run a PGB generated .ipa file on them, since they are a different instruction set. However, I finally figured out a way to use them.
I'm sure most PGB developers already use browsers and the browser developer tools for debugging and responsive design checks. I do as well. I also deploy my app as a WebApp as well as generating native IOS and Android apps using PGB. This is the key to using the Xcode simulators. Simply fire up Xcode and select the simulator (Xcode->Open Developer Tool->Simulator). In the simulator open the desired device (Hardware->Device-><OS>-><device>) and let it boot. Open Safari in the simulator and enter the URL to your WebApp version. Select the "share" icon (box with up arrow) and select "add to home screen". Now get out of Safari and select the new home screen icon you just created. You now should have a good approximation of your app on the selected device. You may need to add meta tags like:
<meta name="apple-mobile-web-app-capable" content="yes" />
Now you can set up your desired app store page examples and use the simulator to take a screen shot (File->Save Screen Shot). Be sure the window scale (Window->Scale) is set to 100% or the screenshot will not have the correct size. It doesn't matter if displayed screen is too big for your computer screen.
Hope this helps!