Before you upload the level, go into the level folder's files and replace the screen.png file with the image you want. DO NOT GO INTO THE LEVEL AGAIN, as that will generate another automatic overlay image. Then simply upload the level or campaign with the upload button.

Oh thank you, I saw people telling me to download stuff and all but that makes it SO MUCH easier

I actually noticed your a campaign creator how cool, I actually downloaded one of your campaigns but haven’t tested it yet