Hi Jackichan,
Replace thumbnail code with the following code
<div class="thumbs">
<?php for ( $i = 0; osc_has_item_resources(); $i++ ) { ?>
<a href="javascript:void(0)" title="<?php _e('Image', 'eclassi_plus'); ?> <?php echo $i+1;?> / <?php echo osc_count_item_resources();?>" onclick="showimage('<?php echo osc_resource_url(); ?>','<?php echo osc_item_title(); ?>','<?php echo osc_count_item_resources();?>','<?php echo $i+1;?>')">
<img src="<?php echo osc_resource_thumbnail_url(); ?>" width="75" alt="<?php echo osc_item_title(); ?>" title="<?php echo osc_item_title(); ?>" />
</a>
<?php } ?>
</div>
And add the below code in your theme js file global.js at the end
function showimage(url,title,itemcount,current){
//alert('jello');
document.getElementById("largephoto").innerHTML='<a href="'+url+'" class="main-photo" rel="image_group" title="Image '+current+'/'+itemcount+'"><img src="'+url+'" title="'+title+'" alt="'+title+'"></a>';
return false;
}
I hope this is helping and it works for me