Awesome plugin, first of all.
I'm just having trouble calling the default thumbnail image, for when the feature image doesn't exist. I'm not sure where it gets uploaded to (it doesn't show up in my Media Library), but the source link seems to be broken.
When I inspect the code, it says the location is in "/files/2012/12/default-img.png" which doesn't exist in my directory. I'm using Amazon S3 to upload media files, so I'm not sure if that info helps.