@xd1986k - If the zip/rar is a solid archive of images, calibre will convert to cbz/cbr when you add the archive, if you have Preferences->Add Books->Read metadata from file contents rather than file name checked, then the first image will be extracted as the cover.
Also note author and title will be always taken from the file name for solid archives that contain nothing but images.
Otherwise you'll have to drag 'n' drop the relevant image out of the archive to say your desktop, and then from there to the book details panel where I assume you see a brown cover. You probably wont be able to drag 'n' drop directly from the archive to the calibre book details panel.
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