I have already set up one custom pane, as per this tutorial https://docs.textadventures.co.uk/quest/custom_panes.html (to display health among other things). I would like yet another separate pane to display an image that can be changed with room or object scripts. I am not sure how to go about this when custom pane generates only one pane. Any help would be greatly appreciated!

My first thought would be asking if your health/etc pane could be done using statusattributes. This is an optional pane which by default can be configured to display a list of the player object's attributes. It sounds like your first custom pane might not be too hard to create using that existing system, leaving the custom pane for your pictures.

If you want more flexibility… well, there's nothing stopping you from having a single status attribute and putting a whole bunch of HTML into it so that it makes the statusattributes pane behave as a second custom pane.

Actually creating a second custom pane is possible (there are threads on the forum from a few years ago discussing how to do it), but it's quite a complex bodge to make it work. So I would suggest trying other options first if you can.