Is there any way to choose an image to use as the cover of a particular comic. I know about the edit comic dialog with the arrows. However, I want to set custom covers. In order to do this, I'd have to decompress a cbz and add in the image manually. This normally wouldn't be a huge deal, but I store all my comics on Onedrive, and in order to actually do that I'd have to undergo to the arduous process of downloading the files from the cloud. Is there any way to just set a custom image to be the cover?

No, not yet.