If you mean a CMS Static HTML Widget then just add a normal HTML image statement like this:
<img src="xxxxxx" />
Just replace the xxxxxx with the path to the image. You will need to be concerned with the image size as a widget size can be restrictive
Take a look at my site. The widget on the left is the CMS widget that I display on most of the pages and the one on the right is a Forum Block HTML widget. The CMS widget has nine clickable images
www.pjksoftware.com