I am running:
XOOPS Version XOOPS 2.4.3
PHP Version 5.2.6

and have installed:
eXtGallery Version : 1.0.8

and all is well. untill.......

1) I downloaded "extgallery-extention-hook.tar.gz" from the link here and uploaded the files included in that tar ball to:

mywebroot/class/textsanitizer/gallery/.

2)I then downloaded the lightbox2.04.zip.

from here:
3) all files in js/ folder went into:
mywebroot/include/.

4) the css file went into:
mywebroot/include/. also.

5) all the images from the image/. file went into:
mywebroot/images/.

6) In the head of my mywebroot/themes/mytheme/theme.html, I added this

"<link rel="stylesheet" href="<{$xoops_url}>/include/lightbox.css" type="text/css" media="screen" />
<script type="text/javascript" src="<{$xoops_url}>/include/prototype.js"></script>
<script type="text/javascript" src="<{$xoops_url}>/include/scriptaculous.js?load=effects"></script>
<script type="text/javascript" src="<{$xoops_url}>/include/lightbox.js"></script>" (without the " at the beginning or end)
I made sure this was BEFORE "<{$xoops_module_header}>"

7) I then cleared my mywebroot/xoops_data/caches/smarty_compile/. folder EXCEPT for the index.html file.

not working at all.

Any Clues?

You also have to modify the file :

mywebroot\class\textsanitizer\config.php

Add a configuration line in the array :

"gallery" => 1;

Hope it helps