I found a simple way to change image depending on the active link too, here is a piece of my code form “theme_primarylinks” function (I ave a PrimaryLink folder in my images in there I have a menuname.png image and a menunameActive.png image):

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// Initialize fragment and query variables.
$link['query'] = isset($link['query']) ? $link['query'] : NULL;
$link['fragment'] = isset($link['fragment']) ? $link['fragment'] : NULL;

if (isset($link['href'])) {
$linkactive = “”;
// should this button be active?
if(substr($link['attributes']['class'], -6) == “active” ) { $linkactive = “Active”;}
// if an active image exixts use it!
if(file_exists(”/opt/lampp/htdocs/” . path_to_subtheme() . “/images/PrimaryLinks/” . $link['title'] . $linkactive . “.png”)) {

$output .= l(”",
$link['href'], $link['attributes'], $link['query'], $link['fragment'], FALSE, $html = TRUE);

}
else {
$output .= l($link['title'], $link['href'], $link['attributes'], $link['query'], $link['fragment'], FALSE, $html);
}
}
…
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