This idea has been merged into another idea. To comment or vote on this idea, please visit LATER-I-294 replace the image in a scheduled post.
It seems like such a basic feature, I thought I was just not noticing how to do it, but apparently no. To change an image you have to delete the whole post and start again. It would be SOoooo much more time efficient if it was possible to just change the image scheduled in a post.