First you have to decide where (in which topic and so) are you gonna post it!
Then.. go to some topic and down that page... there you will see a little square which becomes bigger when you click in it..
under the square it says two things.. you have to click "Advanced editor" button..
Then it throws you on the news page.. you have to go down the page a little bit and you`ll see a word "Attach image" and before it a little white square.. you have to tick it and then you search for files from your computer.. click "upload file(s)" and "submit post" :)

I hope this helps you!!