Yes, yes, I get the drift

But
a) The people who don't realise their pics are too big are hardly likely to be able to post thumbnails. Even if they did they would probably be 2000 pixels wide.

b) It's a lot of fart-arseing about for people like me who don't use auto-resizing sites.

Petem's point about slowing down the forum is probably quite good. However only initial validation would be needed. I don't think we have many malicious wide-posters on here. It mostly happens through not realising either the picture size or the effect it has on the forum window.