Chad Perrin said:
The statement that "we" already have this capability built into "our" browsers assumes some things about visitors to the site that are not necessarily true.
Well you had to dig pretty deep for a not-very-convincing example. :)
Besides, anything less than a one-click method of blocking all images is unlikely to make blocking all images palatable to anyone using a graphical browser. As such, I think that addressing the matter from the server side is rather important, in the long run.
I agree that, at most, it's a nice-to-have. But really the users should be taking this issue up with the designers of their browsers, not the producers of content. http is not a push medium.