For some of us who only have 800x600 screen resolution (sorry, laptops can't grow extra pixels!) wide pictures are highly inconvenient. Signature pictures aren't a problem (yet) but pictures that some people post on the forums are a screen and a half wide. Whilst this might be OK for resolutions like 1024x768, on 800x600 it means I (and others) have to scroll across to view the whole picture. I don't mind scrolling for the picture, but I do mind having to scroll back and forwards to read every single sentence on the same page.
When posting, I think image size should be considered in terms of its screen dimensions and file size. By all means, post images, but consider the size and number of them before you do. If it's going to be too big or you don't know how to make them smaller, then just provide a link to the website so if people would like to see them they can do so themselves.
(PS. I'm not a moderator - these are just my own thoughts!)