Hi Sasha,
If you scroll down to the bottom of your post you will see a box marked "Additional options" just asThe Ferry Man says. If you click on this you can then chose one of your pictures from your files and add it there. If you click on "Browse" you will see your own documents there from which you can chose a picture. You will note that there is a limit to A) the size of the picture and B) the number of pictures or files you can add to one post, which is 5. Try it and see. It is very straightforward and easy to do.
Angus.