If you've got a picture which is already accessible on a web page (in other words not just on your own computer) then you can use img tags in your post like so:
You can usually get the link information from the URL bar of your browser when you are looking at the image, or by doing 'Copy link location' in Firefox, I don't know about IE. That's the string you'd paste into the img tags above.
If the picture is currently on your own computer than attaching it is the easiest way, since that copies it up to this site. Alternatively you could see if your ISP gives you some web space to upload it to, or you could use a third party site like Photobucket to store the picture and link it from there.