To make things clear, I added a screenshot of the city builder Cities: Skylines down below. If you click the image, you will get redirected to the tag Cities: Skylines on Number13.

The corresponding HTML code looks like this:

<a href="https://en.number13.de/tag/cities-skylines/">
    <img src="https://www.number13.de/content/images/2017/12/20160719232251_1.jpg"
    title="Click the image, to see all posts about Cities: Skylines!"/>
</a>

Right after href in the first line you will have to paste the link, to which the user is going to be redirected. The adress of the image that is clicked, is pasted after src (source) into the <img> brackets. Furthermore in the <img> brackets, I also added the attribute title. Here you can enter a text that is displayed, when the mouse is hovered over the picture.