No image thumbnails show up in Media Library after using New Blog Template plugin

I noticed that after a new site is generated using the New Blog Templates plugin, the image thumbnails in the media library for the newly-created site are not visible. The image file is there, but the thumbnail looks blank.

I also noticed that this happened recently after using the Domain Mapping plugin. All the images are also not visible.

So my questions are:

1. How can I have this not happen

2. Is there an easy way to "refresh" the media libraries across my entire network?

Thanks, Joe.