I don't see the need for this right now especially since the
server will probably know where the image is headed (it probably
gave you the document that in-lined the image in the first place).
If you had such a feature, I'd presume it would be some parameter
on the "Accept:" header, but it seems a little bit abstract for
such a purpose.
Besides, we barely have Accept: headers that say the right thing
never mind many servers that pay attention to them not to
mention any of the parameters.
On the other hand, if someone puts "Accept: image/gif; depth=1"
into their browser, I promise to have at least one server which
uses it!