I see, so you're not trying to display the value of a variable inside dynamic content, you're trying to refererence a bespoke and re-usable unit of content.

In which case, you need to use a Markup Asset. Within the Site Map Inspector, switch to the Asset Browser panel and make sure you have the Markup tab selected.

Like you would when adding an image asset, use the hamburger menu in the top right of the tab to Create a New Markup Asset. After naming the asset, place the markup you want to render in your content block in the editor, and Publish your changes.

In your model, place the following inside the content block's question text:

<%# ASSET(MyAssetName) %>

This should render your content. Please let me know if you have any further issues.

Kind regards.

I have not really structure the html for the image as I need to see if it can work first. So what I have done was creating an identifier called "VarHelloWorld" and enter the value below 

<span style='color:green;'>Hello World!</span>

Then I put the code below in the Content Block question text

<%# KEYWORD(VarHelloWorld) %>

OK, would you mind pasting the syntax you've used to try and display the image by its keyword, please?