A user tried to paste a screenshot for a product of mine in the forums and received an error when they tried to post. I tried to paste an image in a reply and received the same error: "body: expected text between 1 and 20480 characters"

Lets see if I can paste an image here and post it: 

Well it didn't make an error but it also isn't showing the image. The forum where we can't post if images are in our posts is at: https://itch.io/t/1066127/failure-to-download-the-trial

It looks like you can't paste images into the post's text box, it may appear that the image is there, but its not uploaded when you post. The only way to put images in the post is with the Add Image button.