Or you can simply hit the picture button that is the second one from the left on the second row of buttons in the post reply window. If you hold your mouse over the button, it should say "Insert Image" before you click on it.
By the way, masoudfun1996, you need to watch it with your posts in the Fan Art threads as they have not made any sense and have been completely off topic. Please check the rules and try to contribute to other parts of the forum as it seems you just want weather icons and don't want to get involved with the rest of this community.