The issue is I am constantly downloading images and then attaching them to the cards.
You can drag an image from another webpage onto a card. I have only tested this behavior in Chrome. This would save you the step of downloading the images.
I want to use this image as the "Cover photo" for the card on the regular board.
Unfortunately, because we resize the images on the server, card covers will be of the image when you uploaded it - changing the image that resides at that url will not do it.