Apple is rather strict in the size of these screenshots. Assuming you don't have a MacBook available to you, you can use online products like BrowserStack but these are not free. However, you can also automate tests on a bunch of devices. If you build through MS AppCenter there is also a (paid) test suite available.

However, in my not so humble opinion, if you develop for iOS you need a Mac, whether you like it or not. You can then run your app in simulators on the Mac and take screenshots from the simulators. I have one, for colleagues who don't need it often we have a Mac mini at the office so they can take screenshots or distribute from that machine.