Honestly, people who use others' artwork without permission or credit really get under my skin. If you don't know who the artist is then don't repost, use, or edit their work be it for cover art or otherwise. It's because of this that artists become very untrusting of others, and most of the time stop creating altogether. A quick Google Image search would easily help you find the artist if you really want to use their art. Search through Twitter and Tumblr if you have to and if you can't find them then search for another artist or create something of your own.
Whenever finding an artist you like, it is always important to ask for permission to use their work or even better for you to commission them for what they do. Every day, they go through with taking the risk of posting their work out of fear of it being stolen and/or used without their knowledge. Lookout for artists and keep them in mind before you even consider using their work. They work very hard to provide fandoms they love some pretty amazing content or even just providing original works they take pride in. Whatever it may be, they've put the time and effort into their art and deserve the decency of being contacted to find out whether or not you have permission to use it.
Have a nice day. :)