I've noticed for a while that Mail.app seems to insist on displaying image attachments inline. This is fine for small web-optimised images, but becomes intensely annoying when you are trying to read several emails in succession, all of which have one or more large (1MB+) image attachments, because Mail takes several seconds to open each email due to the huge image that it insists on displaying inline.
I had always intended to figure out a solution, but today when I actually sat down and looked for one, I had nasty surprise. There is no solution. Apple really needs to sort this out, it's enough to make me consider ditching Mail.app.