For a future project, I'm thinking about implementing an additional panel amongst the side panels : one that displays a simple pixel-art image that depicts whatever room the player is in: I'd make these myself, and would want them to switch based upon the room that the player is presently in (or other variables).

I... have no clue how to go about doing this, really! So any advice & instructions would be much appreciated.

Like this? https://docs.textadventures.co.uk/quest/ui-custom.html

Not exactly: that's just a visual change to preexisting, default panels : I'm looking to add an additional, separate panel to display an image on the side.

You can do it in the same way mentioned in that post, by changing the game CSS/HTML. There are plenty of answers to your questions after that, which should be an easy look-up.

You can change the visual appearance of preexisting panels as listed in the link: it does not go into new panels entirely, or how to have aforementioned New Panel change in response to location and/or variables.

Help from others would be much appreciated!

Hello.

This might help:

https://github.com/ThePix/quest/wiki/Advanced-UI-Part-02:-New-Status-Pane

Oops! I posted the wrong link.

A custom pane would probably be better (and easier):
https://docs.textadventures.co.uk/quest/custom_panes.html

I know I previously posted a chunk of JS which moves the image panel into the sidebar. If you want it to change for each room, I suspect this would be the simplest way. Though I haven't managed to find that thread on the forum yet.

I did save this forum thread about adding an additional image pane on the right side:

http://textadventures.co.uk/forum/samples/topic/q5zedoe560au_ii73oamoq/adding-graphics-in-a-specific-location-gui-solved

Since I'm the one posting the thread the Dcoder linked (above) I can verify that it works like a charm.
If you want to see it in action; the game can be found here.