The title says it all. I took a look into GitLab docs but couldn't find clear-cut solution to this.
How do I add image to readme on GitLab ? Image that's within the repo.
From the docs
Inline-style (hover to see title text):

Reference-style (hover to see title text):
![alt text1][logo]
[logo]: img/markdown_logo.png "Title Text"
images/)BEST AND EASIEST WAY:
Use HTML element like this:
<img src="https://github.com/user/repo/blob/main/output.jpg"
align="center" height="350" width="600"/>
This will allow to control Height/Width/Alignment too.
img tag really is the best way to control the dimensions of an image within a Markdown file.
gitlab.com/gitlab-org/gitlab/-/issues/22822.