I think I'm missing something here in the implementation. The way the plugin is built is to first dig out the featured image and if that is missing then you move on to other methods.
As far as I understand, WordPress allows you one size of the thumbnail image, which is ideally what you should fill in the Top 10 settings page.
I've noticed that if these don't match then you end up with WordPress serving a full size image instead of a thumbnail. I could be wrong on this one.
There is also a plugin filter which uses timthumb to resize the images, which I believe could be overdoing it. I can test the implementations on my blog without the filter.
Am happy to implement a universal solution. Any suggestion on this?