nope. that won't be enough.^ Nope. It's only necessary to allow loading files only from sites with known thumbnail rules (like own attachments or twimg, for example).
even attatched pictures can be very large.
As i said, this addon will just grep the first image in the post.
if the user would have used thumbnails, it would only load the thumbnail. if they have attached the whole 10MB preview, then this one will be loaded.
All that can be done would be forcing to use thumbnails.
Only loading pictures from some resources would result in a lot of whining from users not being able to hover over the pictures :/