Yes, the .sig limit is posted right on the screen where you change your .sig, which is why the Knights are now telling me when someone's .sig gets too big instead of PM'ing the person with the oversized .sig.

In order to link to an image from a site you don't run, just use the img src tags Shlup posted; You paste that in the textarea where you edit your .sig (Control Panel -> Edit Profile -> Signature)