Hi. Please abstain from converting JPG images to PNG without a good reason, like you did here and in multiple other instances. It is against the guidelines and it does not take much to edit an article and change the cover file name to .jpg, then upload.
Topic on User talk:Jenya
Hi, for some reason it wasn't possible to replace PNG covers with JPG ones back then. Not sure if it's still the case.
That is correct, but it's possible and encouraged to newly upload JPG files and then simply edit the articles to change the file extension of the covers.