Hi themixx, I think it may be a theme issue - give another one a try to confirm it. HTML syntax highlighting is supported in Obsidian
Markup - markup, html, xml, svg, mathml, ssml, atom, rss
By tha way, you can check the full list of supported languages
Edit mode: Verify your code block is exactly like this
```html
<img src="cat.jpg" alt="cat">
```
Preview: you should be seeing something like this

Hope this helps you!