You are allowed to post images, as long as they are hosted elsewhere. As noted, just paste the link from the image hosting site.
This is probably a variant of the same bug that is being discussed here.
We don’t allow users to upload their own videos and images to our server (we’re not a hosting site). A few months ago, the Discourse folks made some sort of software change, and now when Discourse generates its preview for a link, it gets confused and thinks that the link preview is somehow you trying to upload an image, even though you aren’t.
The Discourse folks have been made aware of the bug, but so far there has been no fix for it.
There are some workarounds for youtube videos in the other thread.
Since the other thread is focused on youtube videos, please post video issues and workarounds in that thread so that we do not have duplicate thread topics, even though this is probably the same bug. Use this thread to test images and post workarounds.
As was already suggested, please post the link that you are having issues with. You can break the link by adding a space between the http at the beginning of the link and the rest of the link. If all else fails, send me a PM or an email with the link.