@Korina I'm still in the two weeks trial period offered by Google Play, but it will pay with my PayPal account afterwards.

You can see that you can add PayPal as a payment option through Google Play in this screenshot:

Google Play subscriptions allow you to pay with PayPal - I believe it's under "additional payment options" or something like that.
You can pay using Google Play if you buy Premium through Windy's mobile app.

You can use a 3rd party app to do this with Windy, such as Wallpaper Engine. Just click "import from URL", copy over Windy's URL and you're done!

Note: you may want to rearrange your desktop icons since this will essentially be a webpage on your desktop.