I find one cover option I like using a Google Image search, then I use Google Image 'Search for the Image' function to find more covers exactly like that one so that I can pick the cover size and link I like best. My favorite goto are any images posted on Pinterest since those are most likely to remain online the longest.
Alternatively, I search for an image based on the information about the comic, title, issue number, date, and/or artist.
Alternatively, I scan a cover from my own collection and post it. As long as the cover meets the minimum requirements for actually being a comic book cover [Graphic Novel, TPB, etc.] then it's fine