Hi Michiel, sorry for taking so long to respond,
For Lightbox integration with Exhibit,
1. download Lightbox and put the folder with the javascript somewhere in your server and refer to it in your header. Just add the following lines to your header.php, before the </head> tag:
<script type="text/javascript" src="http://www.website.org/js/prototype.js"></script>
<script type="text/javascript" src="http://www.website.org/js/scriptaculous.js?load=effects"></script>
<script type="text/javascript" src="http://www.website.org/js/lightbox.js"></script>
2. Add the Lightbox CSS. I copied everything in lightbox.css and just pasted it at the end of my WordPress stylesheet. You could also add a line in the header as suggested in the Lightbox docs:
<link rel="stylesheet" href="css/lightbox.css" type="text/css" media="screen" />
3. Upload the Lightbox images folder (I put it in the root folder)
4. You should test it by manually adding a Lightbox link in a post to see what happens. Such as (from the Lightbox docs):
<a href="images/image-1.jpg" rel="lightbox" title="my caption">image #1</a>
or better, try two or more images and putting them in a slideshow, like this:
<a href="images/image-1.jpg" rel="lightbox[slideshow]">image #1</a>
<a href="images/image-2.jpg" rel="lightbox[slideshow]">image #2</a>
<a href="images/image-3.jpg" rel="lightbox[slideshow]">image #3</a>
If this works then you're nearly there.
5. Here's the trickier part. Edit the /wp-content/plugins/exhibitimg/exhibit10p.php file. Around line 371 there is the following:
$ret .= "<a href=\"{$imgurl}\" onclick=\"exh_popImage('{$imgurl}', '".addslashes($image->caption)."'); return false;\">";
Change it to:
$ret .= "<a href=\"{$imgurl}\" rel=\"lightbox[slideshow]\" title=\"".addslashes($image->caption). "\">";
Then you should get all of the images in your post in a nice little series.
Now, I would really like some help in modifying Exhibit to use intermediate images, with a link to the full size ones. If anyone is interested in what I've done so far here, let me know.