Corrections/additions: [edited to add 2 more things]
"Jpegs, gifs, and other images can not be posted in a quick post. You must either start a thread, be quoting to another post, or editing your own to access fonts, post images, add smileys, and do things besides post text. Quoting is the most common method."
OR, you could click the "(RICH TEXT REPLY)" link down by the "Post a quick reply" box.
There is also another way to quote from other threads, if you want it to look the same as a quote from the same thread: Just quote in that thread (unless it's locked and you therefore can't quote), select all (click in the reply box and Ctrl+A), and copy/paste it into a rich text reply box in the new thread. (Don't forget to close/cancel the other one!)
Also, the quick post can be used for things beside plain text if you have the HTML. I'm not educated in HTML but I know the basic stuff, like i /i and b /b, and for more complex stuff like blackout text I have some examples in a Notepad document so I can just paste in the code if I want to. Turns out you can add images with img /img that way too, according to my notes.