Ok, the problem is that, if you don't specify that a link is an image with the < img /> tag, discourse try to figure it out by looking at the extension that in this case is
.png#excluded so not a valid one...
There are two way to workaround that:
1. Use img tag
<img src="http://imgur.com/yourjokerimage.png#excluded" />
2. add ,extension at the end of the excluded link
Neither one is intuitive, so maybe it's better if i add a way to exclude the entire post by specifying #excluded on the bottom