You do know that many browsers automatically resize graphics so you can’t necessarily tell what size it is, right? If you just cut and paste the URL, you can’t always tell what resolution it really is. And I say that as someone who uses iOS Safari.
the original graphics size is easily knowable information. or, simply make it a practice to post through tinypic, which allows you to resize on the fly. easy peasy.
hot linking is pretty perilous for a lot of reasons, beyond just posting criminally large graphics.