This issue is discussed at length here: https://github.com/markdown-it/markdown-it/issues/38
It seems you'll just have to live with needing to wrap the url in < and >
The topics thumbs using a link are not resized/cropped resulting in weird issues. Is this something normal (like to have big backgrounds) or an actual bug ?
I think that in ACP since you can enter a max value it should be cropped like when you upload the file.