The exact thing I looked for

MovieIcon just works like a charm. It is very easy to use and and found me every icon i searched for. I havent seen anything like this app on the net. It is very useful, especially if you have mkv files, that you can’t just edit with the usual programs. At first I wasn’t sure about buying the app, because of the mixed reviews, but I had no problems whatsoever and the problems mentioned seem to be resolved. The price may seem a bit high, but the dev has obviously put a lot of effort into this and it is worth every cent. (I just wish, that there were any app like this for audiobooks.)

maxi 132 about MovieIcon - Adds cover art to your movie files, v2.9.3