acub

in case anyone else needs this: i’ve solved it.
because i didn’t need both [img] and [caption] image, i just modified
quicktags.dev.js like this (around line 379):

function edInsertImage(myField) {
	var myUrl = prompt('quicktagsL10n.enterImageURL', 'http://');
	if (( myUrl !== 'http://') && ( myUrl !== null)) {
		var myTitle = prompt('quicktagsL10n.enterImageDescription', '');
		var myAlign = prompt('Insert image alignment\n( left | right | center | none )', 'left');
		myValue = ' [caption width="200" align="align'
				+ myAlign
				+ '" caption="'
				+ myTitle
				+ '"]<a href="'
				+ myUrl
				+ '" title="'
				+ myTitle
				+ '"><img src="'
				+ myUrl
				+ '" width="200" alt="'
				+ myTitle
				+ '" /></a>[/caption] ';
		edInsertContent(myField, myValue);
	}
}

Of course, would have been nicer to have 4 radio-buttons for selecting the alignment, but since I’m not much of a programmer and couldn’t find a proper example, they’ll just have to type in the alignment. It’s way better than having them type the caption/a/img list of commands.
This is for users who really don’t know their way around HTML.

If anyone has a better idea of how to do this in a more elegant way (plugin-like, so it stands to WP upgrades) I’d be quite happy to hear about it.

Cheers.

I guess I found the answer here.
Cheers.