My website went through a huge transformation from multisite with 11 sites to single installation. The IDs of the posts will not match the images on the library resulting on wrong images being picked, it was a mess! I sent a support ticket and Martin tweaked the plugin to work regardless of my messed up installation, now i've set the correct images as featured for more than 3000 posts in different custom post types, the plugin also can delete the first image of each post which is very handy if your theme automatically puts the featured image on top. Incredible plugin with incredible support.