Can't see image in modification mode

Hi,
first thing: thanks for the plugin!

I have a problem: when I activate the plugin and I insert the code in my function.php file template, if I want to modify some image, I can’t see it in modification tools.

You can see an example here: https://img593.imageshack.us/img593/3139/modimagemedia.jpg

Can you help me?

Thanks in advance!

https://www.ads-software.com/extend/plugins/multiple-post-thumbnails/