VK API - community cover download. How to implement?

Question

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Good afternoon. I am June, please do not throw slippers. This is my first time working with API as such.
There was a problem, I thought I'd solve it, in any case I'll gain experience.
I want to upload a cover art (which is 1590 by 400) to a community/group where I am the owner.
Wrote the download in php, in the end, everything is fine, but ...
1) The cover does not fit through photos.saveOwnerPhoto, i.e. the one that is 1590 by 400, the answer swears that it is large and needs less, in other words, it turns out the avatar of the group.
Q: How do I download the cover then?
2) After I uploaded the picture to the server, I got 3 variants of it and photo_hash. And then the question crept in - what's next? The image didn't load as expected.
As far as I understand, I need to somehow approve the replacement of the picture through JS, but this algorithm is already incomprehensible to me. I have a regular standalone application, I upload a picture from my local server.
Question: In short, what to do next after filling the picture, how to insert it?

Answer 1

When will you learn to google! First link in google :)
ru.stackoverflow.com/a/614998/219079 Explained
everything there...