The picture has to be stored somewhere with access to internet (hosting). FSM will not host your picture. You need to find a host for it, unless you have your own web page (or something like Facebook). The most common photo hosting site for folks who post in this forum tends to be Photobucket.
You post the picture on a site like that, then determine the web address of the photo. You then click on the picture logo in the tool bar at the top of the reply window. The photo icon is in the bottom row, just below the underline font icon- the rectangle with a mountain and sun in it.
One advantage of Photobucket is that it is easy (click on photo) to get the direct web address of the photo.
BTW, there is a help section of the forum that specializes in posts relating to problems such as that.