Advice in Creating a Network of Template Sites: Use Shared Images or Each Blog have Seperate Images?

Hi Everyone - I'm just looking for some more experienced advice on the topic of content delivery in a large network. Currently I am working on building a templated site that will then be customized per client, but still maintain much of the same content (articles, pics, etc.). My question is with regards to the images:

Should I link to a single shared folder of images for each site on the network (ie. so that each site on the network is pulling the same one image file) OR does it make more sense to simply duplicate each site, including the upload folder and have each site reference their own copies of the same image?

Which option will maintain the best server speeds, etc.

This is my first run at building a network this large and so I am trying to make it as scalable as possible.

Cheers.
dale