Should be cover. This image has class "content-post-image". "It is obligatory to use a cover image: If the image is present on the page and described in the source as “featured-img”, “cover”, “lead_img”, “main_image”, etc." https://instantview.telegram.org/checklist#6-2-2-cover-images
i am not see any class which says on checklist so it is not critical and it is optional to set this image as cover :)
Type of issue
IV page is missing essential content
Reported
Mar 17, 2019
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If the image is present on the page and described in the source as “featured-img”, “cover”, “lead_img”, “main_image”, etc." https://instantview.telegram.org/checklist#6-2-2-cover-images