How do I get an image to pop up when I mouse over a word (text)?

Basically, the title says it all. In the body of my text I want an image to pip up when you hover your mouse over a certain word. I know this will require some custom css, I just can't find it!

I'm looking for a similar capability: I'd like a brief definition to pop up when someone hovers over a highlighted term; kindof like a "tooltip" in applications software...

How to do this?

Use HTML code

<a href="#">Required text<img src="image url" /></a>

Use CSS code

a img { display:none; }
a:hover img { display:block; }

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Thanks for the suggestion; I'll give it a try. Though I've used html and css before coming to Squarespace, I don't know yet how to "get to" the html and css yet. I need to watch an instructional vid or two before trying.