To image sharing websites (many around; search via google to find them or use your own personal website). You can not upload them to this website (the trainz forum); you will have to host them elsewhere.
If you have the URL of the image, you add that between IMG tags. Or use the tiny image icon at the top of the "quick reply" interface and you get an popup in which you post that URL.
Some image hosting websites even offer pre-made links / tags / code-snippets that you simply have to copy-paste.
Just use Imgur, it really easy to use for hosting your photos. I don't think that they delete photos after a lengthy period of time. http://www.imgur.com
I thought it might have been the first screenshot taken by each of us, regardless of when we started. Could have be interesting seeing a comparison between members first attempts in Surveyor and the date they started, compared with what can be achieved with time, patience and experience.
Just use Imgur, it really easy to use for hosting your photos. I don't think that they delete photos after a lengthy period of time. http://www.imgur.com