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

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

And there it begins. Wich *box should be supported, wich not?

In the first place, I don't know in how far those different solutions actually differ in the parameters they expect in the HTML code. Maybe there is no difference. I don't know. Since such a Lightbox software is just a pack of Javascript stored on the Server, there is no need to adapt to various software packages that might be installed on the client's computer.

If the various Lightbox implementations expect different attributes to the A and IMG HTML tags, MLF might decide for a single solution, or it might make that configurable for the Forum operator. I don't care. BTW, I am not a forum owner, I am a simple user. Please note that all these Lightbox implementations rely on "nonobtrusive Javascript", i.e. don't have any Javascript code inside the HTML BODY. The whole thing is just loaded by the SRC attribute of a SCRIPT tag in the HEADer.

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.

That's possible but it complicates the task to embed an image. It's not a passing by action to embed an image anymore, when I have to provide additional informations.

The description which would appear as TITLE to the A tag, and identical as ALT to the IMG tag, and as CDATA of the FIGCAPTION tag, is the same text which the forum user would type into the message before or after the image which she or he inserts in full size. A FORM with two FIELDs, one for the URL of the image, and the other for the caption to the image just changes the way, the latter is entered, but does not require anything additional.

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.

A question: How much users will use this function in a forum?

I hope that all those who currently bloat their posts with full size images would use that. And as far as I can see, this is increasing.

Have a look at this: http://www.ice-treff.de/index.php?id=325387
Or, in another forum (powered by a different software):
http://www.bahninfo-forum.de/list.php?9

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

No, that's to much, to overcomplicated. The software should handle this automatically. If you choose [img=thumbnail] the script should choose the correct image and link the other correct image.

Well, this third FIELD is optional. Have the text over this "I already have a smaller version of the image to be used as thumbnail", and allow to enter an URL there could save a lot of transmission time for the user, who would have to get the full size image, just to be compressed to thumbnail size. Maybe this user is not interested in viewing the full size image, but has to wait for the transmission of the full image with lots of megabytes of date going over the wire just for nothing.