Where can I find the theme’s default sizes for banners and sidebar images?

Hi, I’d like to avoid blurry banners like the one here at the top of the page when you open it on mobile: https://devenirbilingue.com/apprendre/anglais/application-logiciel/avis-duolingo-test/
For this one, I used a 320x100px which is (I think) a standard large mobile banner, and I thought that the theme would not resize it to show it on mobile.
What format should I create to get a nice and clear banner, and is there somewhere a list of default sizes for images and banners using Newspaper theme?
Thank you so much, it is an issue I have since a long long time and I’d be very happy to finally understand the answer!

Ditto here

What do you mean? I uploaded the 320x100px file in the library. This file is not blurry when I open it with a Preview app, it only becomes blurry when it’s used on my website. Does the template transform the full-size image using a thumb ?
Thanks.

Yes of course.

I’d like to understand the issue, but also it would help to get a list of recommended sizes for Newspaper template. For example, I always use 700px width for images that illustrate my posts but I don’t know if this is the right format (these images take the whole width of a post with a 1/3 sidebar). Example here with the image of the boy on a computer: https://devenirbilingue.com/apprendre/anglais/cours-enfant/concours-gagnez-3-cours-langue/
Thank you.

I know and I use it when I build my pages. However, I can’t explain why the mobile banner is blurry here whereas I did all what you mentioned : https://devenirbilingue.com/apprendre/anglais/application-logiciel/avis-duolingo-test/