The pages you make (as php developer) could
use client-side javascript to detect the local
screen resolution. Then, theoretically, you could
send an Ajax message back to the server, to set
a session variable, so all subsequent requests
could be either dynamically generated, with differing
image sizes, or you could, maybe, use url-rewriting
to redirect to the right static html (with differing
image sizes).

But that's got a hidden catch22. Because the 37% of
all users who still use 800x600 resolution probably won't
have browsers that can deal with Ajax requests.

Has anybody ever figured out how to optimize image
sizes for different screen resolutions?