Adding the image into the term page is easy, and is done from the featured image metabox, which appears is a WordPress built-in feature. It appears on every page or post editor on the bottom right side of the screen (location may defer if you’re using the WordPress interface with a non-English language).
The size of the featured image can be modified to fit the tooltip or as desired.
While hovering over the term in a post or page, the featured image will be shown inside the tooltip. It is also possible to set the tooltip to be transparent so only the image will be displayed, without the tooltip background, as shown in the image bellow:
Using the same metabox, you can override the plugin settings and disable the featured image from appearing inside the Tooltip. If this option is checked, the image will only appear in the specified term page, and not in the tooltip.https://www.cminds.com/wordpress-plugins-knowledge-base-and-documentation/?hscat=6-cm-tooltip