title says it all

Within your game, you can take screenshots by pressing the Print Screen/SysRQ key on your keyboard.  You can find these screenshots in your screenshots directory under your root game directory.

Alternatively, you can remap that key in your Config panel or get an external screen capture program to get the shot. 

As for uploading, you can't upload the image to Thottbot.  Thott uses tags that display styles, clickable links, and external images.  So what you have to do is upload your screenshot to say ImageShack or Flickr, and then link the path of the image from Thott.  If you scroll below the message box, you will see a list of Thott tags you can use.

[spam] or alternatively use http://xs.to
(clean, fast, free, no registering, etcetc)[/spam]


but yes, what has been said above

make a screenshot
go to such a site like xs.to
upload your picture (browse to your (c:\program files\)world of warcraft\screenshots folder and select the right one)

once uploaded you get a bunch of results
(from xs you can best use the 'direct link' or the 'Website that shows your image')

then somewhere (like on thottbot) you can write the result

either [ url httpWebsite_that_shows_your_image_here]please click image[/url]
or [ img]httpDirect_link_here[/img]

(well, without the space before [url] and [img] )