Stealing is in the eye of the beholder. As Hacker Jack has pointed out, numerous times, anyone who uses a poster from IMPA or any other site, is 'stealing' the work of the person who made the poster. Most people who make covers here do this.
As to 'stealing' someone else's cover, that isn't limited to just using the whole cover, untouched & uploading it as your own, though many have done just that, but it also includes using the back, while you may make a new spine & even use a different poster for the front. The fact remains, if you didn't compose the entire cover from your own research & collection of images, credits & logos, then you're stealing someone else's work.
While the person who made the original cover from images found on the Internet was 'stealing' those from their originators, once they've composed them into a cover, that work becomes, at least within the cover community, an original work. Any part of that original work that is identifiable by the designer, copied & used by another constitutes 'theft'. The degree to which such use extends is immaterial, at that point. Most cover sites will take down such posts, if the original designer/artist complains. It's a courtesy, not a law.
Anyone who makes covers should be using their own images. Yes, many will use the same original studio images, which is why more experienced artists won't use a stock poster, unaltered, to make a final cover. Adjustments, cropping & editing is usually done to make it unique. When these are identifiable, theft is much more obvious. Simply changing logos or stills or altering a spine on an existing finished cover does not make it 'your own'.