i personally couldn't care less about mixed content warnings. i think it's good that browsers make you aware of mixed content issues, but imho, it's not the responsibility of our forums to "fix" these warnings just for the sake of having no warnings display
. mixed content isn't necessarily a problem.
however, i think proxying all external images (and other embedded resources) is a useful feature for privacy reasons. if it's a forum where privacy is paramount, it's a good idea to shield your users from surveillance by any and all monitoring by external servers via client requests. i think it's a shame that such extensions/efforts are presented primarily as a solution to mixed content warnings -- the real solution to this 'problem' is user education about what these warnings actually mean.
and yes, i do think that having an extension that rewrites links in posts (if possible) like the EFF's HTTPS Everywhere
(not 'anywhere') tool is a great idea. if there is a https version of a link/resource available, it's probably in your users' best interests to use it. but it's not a replacement for image proxying. https-everywhere and image proxying are solutions to two related problems and could be used separately or in tandem.
also potentially of interest to some forums: removing
from request headers so that when users follow links to external sites, the external site isn't aware that the user came from your forum. implementing this correctly however could require a redirector on a separate domain. that way, i think you can take care of