[img][/img]
Those are image tags (which I've surrounded by code tags so that the text appears), popularly used in forum software. Any full URL of an image's address will be rendered inline in your posted message.
So like AcrossFromHoss said, upload your image/get it hosted somewhere (photobucket.com is another example of a free service), get the image's full address from http through .jpg and paste it into image tags. (Or highlight the URL and click that image button.)
Be aware that free image hosting services naturally want to advertise and so when you copy the picture's address you might get more than just the address. When you paste it in here however you'll be able to edit extra stuff out.
Clear as mud?