Simple Image Gallery Pro loads an additional instance of Mootools, so Slimbox will not work. I editted plugins/content/jwsigpro.php to remove the additional Mootools script:
Look for
$sigheader .= '
<script type="text/javascript" src="'.$mosConfig_live_site.'/plugins/content/jwsigpro/slimbox/mootools.js"></script>
<script type="text/javascript" src="'.$mosConfig_live_site.'/plugins/content/jwsigpro/slimbox/'.$language.'/slimbox.js"></script>
<style type="text/css" media="all">
@import "'.$mosConfig_live_site.'/plugins/content/jwsigpro/slimbox/'.$language.'/slimbox.css";
@import "'.$mosConfig_live_site.'/plugins/content/jwsigpro/sigpro_'.$sig_layout.'.css";
</style>';
and change to
$sigheader .= '
<script type="text/javascript" src="'.$mosConfig_live_site.'/plugins/content/jwsigpro/slimbox/'.$language.'/slimbox.js"></script>
<style type="text/css" media="all">
@import "'.$mosConfig_live_site.'/plugins/content/jwsigpro/slimbox/'.$language.'/slimbox.css";
@import "'.$mosConfig_live_site.'/plugins/content/jwsigpro/sigpro_'.$sig_layout.'.css";
</style>';