But what about a slide show like those made with Lightbox, Slimbox and the like?

[...]

Does "my little forum" provide the better ways, the slide show where the user can move from one full size image to the next without having to return to the list of thumbnails?

No, it doesn't. And –in my humble opinion– it shouldn't. Not, because I don't like the idea, but there are ready to use solutions. You named some of them yourself.

If necessary it's a task for the developer or an interested user to optimise the HTML and CSS code to support the *box-scripts.

That's what I am looking for. Let's have a look at a thumbnail entry that resulted from an image which I inserted, and let me explain what changes I deem to be necessary:

<p><a rel="thumbnail"
  href="https://upload.wikimedia.org/wikipedia/commons/thumb/2/2b/Weener_Bahnhof.JPG/1024px-Weener_Bahnhof.JPG">
  <img src="https://upload.wikimedia.org/wikipedia/commons/thumb/2/2b/Weener_Bahnhof.JPG/1024px-Weener_Bahnhof.JPG" 
  class="thumbnail" 
  alt="[image]" />
</a></p>

I use as the target Lightbox-software Slimbox (www.digitalia.be/software).

The attribute value for "rel" should be specific to each post, so that in the case of a flat display of all posts in a thread (instead of tree), not all images are being shown in one single slide show, but separate ones per post. And the rel value should be "lightbox" instead of "thumbnail", and "lightbox-xxx" with "xxx" identifying the actual post.

Then the forum software would have to provide a query to enter a description or of the image, which would then be presented as value of the alt and title attributes of the a and img HTML tags, and would then be shown in the slide show as caption to the individual image. This query could be a field in a HTML-form which queries both the URL of the image to be shown and the caption/description for it. User acceptance of the slideshow would be low if they would have to switch between the slideshow and the actual post to see the explanation of the image contents.

BTW, it might be useful to provide for the user to supply a separate URL for the thumbnail, if there is already a smaller version of the image available, as is the case for images found on commons.wikimedia.org

BTW, why is the "inline-code" text so small compared to the regular text?

It should have the same font size. This browser is configuret to show regular text at 11 point, and non-proportional text as Courier New at 13 pt size.