A really interesting question, and an issue I had to think about a couple of years ago. I had been concerned that it would have a negative effect on SERP position when I started referencing images from an external source for some of my smaller websites. In reality, I found no problem at all, but my sites were well established.
I would imagine it would have a very slight negative effect, so for example if you were launching a new site, it might be a bit troublesome. But if the site already has enough 'juice' to counteract the negative effects, everything should be fine.
I'd be interested to hear your experience if and when you implement this.