Ah a good old rant......thought I would have my 10 cents worth...
I have spent many years working with all sorts of people from all walks of life and I have to say I have given up (in a positive way) expecting people to do what to me would seem obvious. The fact is human beings don't all see things the same way and in my experience people don't put over-sized photo's on a post, or use (what looks like to me) stupid file and folder names on network shares to p*&& others off, there is generally no intent on their part - so "having a go" at them or "showing no tolerance" does not really get you anywhere, in fact it borders on bullying.
I agree with @IanB, if such an image offends you just ignore it and don't bother looking at it.
The fact is we live a "customer centric" world that today recognises that people are in fact "fallible" so in acceptance of that fact I would have another take on it. Software like this SMF forums software we are using is outdated.....and here is why. Look at some big success stories, Apple, Google, YouTube, Flickr and many others they are good at one specific thing which is empowering users to achieve something great through creating a good user experience, which you would argue would otherwise definitely be beyond an individuals capability. Lets take YouTube, I doubt many people would argue that youtube is anything but a good service right? (if you don't agree with that point you may as well skip the rest of what I have to say as I am likely to seem inadequate in some way!) Well imagine if every user had to upload video in a certain format, size, frame rate, colour space and resolution, well youtube would be no more....what youtube and many other services figured out al long time ago is they need to deal with the complexity so their users don't have to - and thats what makes them so successful. That does not mean that people are stupid or incapable, even if that was true, why should it matter, should stupid or technically incapable people not have the right to post their content to youtube - of course they should.
So back to this forum software, I would argue that if this software handled images properly and dealt with scaling on the behalf of the user (like Facebook does for example) then your problem would go away because your over-sensitivity to over-sided images would not be experienced and the poster would not need to be concerned that your particular sensitive spot is over-sized images and someone else's is image position etc...computers and the internet are here to help us, they are not here to create a tech crowd elitism...
so your complaint really ought to be about this forum software and not the people posting images that look to big IMHO...no doubt not everyone will agree with me but I would expect nothing less - we are all human after all.
Gerry