'cut and paste' text from others is called quoting... you can click the quote button and then the 'magic' text appears...
For your other question (the image one) you should take a look at this page:
http://forum.enjoysudoku.com/faq.php?mode=bbcodeBut one generally advice, only quote text when it makes sense and only quote the relevant text... if your post is right behind the original one then it usually isn't required to quote... (unless if the text consists of several pargraphs and your reply to each of those is rather long, but then quote the first paragraph, add your reply, the second paragraph, ...)
Images are another thing... the problem with an image is that it is stored at an external server... meaning that it could disappear after a while and that could make a thread useless... (for example when you paste your grid as an image. If the image is removed a day/week/month/year/... later then noone will know what you are talking about, which is bad from an archive point-of-view)