Hi! Thank you for using our plugin. It’s always nice to have people around ??
Regarding your specific problem… I just accessed your website (I understand it’s clickart, right?) and I see nothing strange. I downloaded your theme and took a look at its source code, and I think I know why the plugin is not working.
Essentially, the problem we’re facing here is the poor support that WordPress offers when it comes to replacing a featured image. You see, WordPress does not include (in general) any good hooks for replacing that image! However, there’s a couple of functions (the_post_thumbnail or get_the_post_thumbnail) that do offer a hook. These functions directly output (or return the equivalent string of) the HTML code that will display a featured image (that is, the img tag with the proper link). Our plugin relies on a hook they use called post_thumbnail_html and creates a new img tag when required.
Your theme (and others) do not use those two functions. Instead, they use a combination of WordPress functions in which we cannot easily hook. If you look in your theme’s code for wp_get_attachment_url(get_post_thumbnail_id($post->ID));, you should see where the URL of a featured image is loaded and, right after the call to that function, where the URL is used inside an img tag. If you replace those lines with a simple call tothe_post_thumbnail($post->ID);, you’ll get the same effect AND our plugin will work.
