I created a sample repo to reflect my problem:
https://github.com/dakky/sample-support-hugo-discourse
EDIT:
ok I don’t know what was the problem, but it’s partly solved:
On the single page view (http://localhost:1313/blog/2017/06/testtitle/) works fine. maybe some kinde of browser/whatever cache problem?
But the problem, that there are no images on the frontpage persists. any idea?