There is a work around. Granted, it takes some time, but if you really need this functionality, try the following. Make sure your cards are visible next to the note you are working on. Then, look at the image displayed on the card cover. If you don't like it, find it in the note and use the cut function (command + X) to remove the image. Wait to see the card update to a different picture. Then, paste the image back in. You will notice that the cover picture doesn't change on paste. Keep doing this until it "picks" the image you want. For instance, if you had a rose, a lily, and a daisy picture. You want the cover picture to be the daisy. Just keep removing the other images until it "selects" the daisy picture. Then, you can paste the images back in without concern it will change.