bikkun

That works, thanks!
Here is the fixed I used, had to hard code the types and cdn url because w3total stores its config in a text file.

function cdn_image_url($url)
 {
     $types = ["gif", "png", "jpg"];
     $type = substr($url, -3); 
 
     if(in_array($type, $types)) {
         $site = get_site_url();
         $cdn = "{Insert cdn url here}";
         $url = str_replace($site, $cdn, $url);
     }    
     return $url;
 }
 add_filter ('wp_get_attachment_url', 'cdn_image_url');