On Wed, 31 Aug 2011 16:37:24 +0100, Savageduck wrote
(in article 2011083108372416807-savageduck1@REMOVESPAMmecom):

If you resize to 800 x 600, 1024 x 768, or 1280 x 1024 you shouldn't
get too many complaints from most viewers.

As a rule of thumb limiting the long dimension of any image to 1024

Using StatCounter's figures for the last three months (and making a few
somewhat heroic assumptions and simplifications), 98% of users have monitors
which are at least 1024 pixels wide and 68% have monitors at least 1280
pixels wide.

It follows that your rule of thumb will mean viewing pictures at full size
should not be a problem for two-thirds of people; nor should it involve too
much scrolling for almost everybody else.