Raw HTML works
Try the following: after uploading your image, use img tag with src pointing to the path of uploaded image, for me it’s working:
The following is from one of my files
<img src="/uploads/d19fcc3d3b4d313c8cd7960a343463b6/table.png" width="120" height="120">
Check this link, where I’ve put different size of same image https://gitlab.com/Basel.issmail/resize-image/wikis/resizing-image
There is an issue on gitlab discuss Add control over images with Markdown