Some additional requirements

Hello…

I have a page where I use your plugin and the overlay image was working fine. But as I had to switch from hosting service i took the opportunity to switch to nginx in a Centos 7 Server, installed php manually and transferred the site to the new web server. Then the plugin stopped overlaying the post image and thought it would be bug of your plugin. after debugging I discovered that my php installation didn’t include gd library by default, so I think it would be great to have that information on requirements for this plugin.