I have stored a Markdown file and an image file in a Git repo as follows:
readme.markdown
images/
image.png<
I had the same problem with my Logo inside of a readme.md in my Extension and with Gitlab (running on a Ubuntu-Server).
Ubuntu Version: 16.04.2 LTS
Gitlab Version: 10.3.4 56dc722
You can also use a snippet for image-usage in your readme.md by simply following these steps:
If you get the following error you forgot point 5:
The form contains the following error:
Content can't be blank
Now, if you edit the snippet again, you can copy the Image-Code looks like this:

to your readme.md file.
If you like to use a link also, it has to look like that:
[](https://your-destination.com)
Hope it helps other ppls