In "WEBCOMIC SITE/LAYOUT" -> "Edit Layout":
- Search for something like "<a href="/comics/first">First</a>".
  There should be similar menu items like "Comics", "Latest", "Archive",
  etc.
- For a link you want to replace with an image, before and after might look like:
  Before:
  <li><a href="/comics/random">Random</a></li>
  After:
  <li><a href="/comics/random"><img src="files/my_random_image.png" alt="Random page"></a></li>
- Save changes by clicking 'Update' at the button.

In "WEBCOMIC SITE/LAYOUT" -> "Upload Extra Files":
- Upload the image you want to show for the link, such as "my_random_image.png".
If you change the path to some folder, you will also have to change it in the previous link.
For instance, "images/layout/my_random_image.png" will require that you change the
value of the "src" attribute to "files/images/layout/my_random_image.png".