Done... In case anyone would like to do the same (I doubt this is the nicest way to do so, but):
Edit /photoshare/templates/thumbnails/template.php
Search for:
$output->Text( $this->getThumbnailHTML($image['title'], $image['id'], $pos, $folderID, $templateHelper) );
Replace by:
$descimagen = addcslashes( pnVarPrepHTMLDisplay($image['description']), "\0..\37\"");
$output->Text( $this->getThumbnailHTML($image['title'], $image['id'], $pos, $folderID, $templateHelper, $descimagen) );
Search for:
function getThumbnailHTML($title, $id, $pos, $folderID, $templateHelper)
Replace by:
function getThumbnailHTML($title, $id, $pos, $folderID, $templateHelper, $descimagen)
Search for:
$html .= "
 |
";
Replace by:
$html .= "
 |
";
This way when browsing the images in thumbnail style, the comment on each image instead of being "click to enlarge" will be the image description. No clue on what might happen if you have 4 pages descriptions on an image, but I doubt noone does.