Find the pic on your computer. Then upload it using the "insert Image" button in the tool bar above where you type a reply. You want to upload from computer, click "Select Files", find the file. Click "open". Then click "upload files". Now it will show up in the reply when you click "post reply". At this point the picture will be a thumbnail and will have to be clicked to be enlarged. If you want it to be big, open the image, right click the image. "copy image url" (or for mac copy image location). now, edit the post you made and click "insert Image". This time, insert from URL. Paste the URL in the box. Uncheck the box that says "Retrieve remote file and reference locally". Now click ok. your picture is now large and incharge.
If the pic is already hosted on another site (facebook, another forum, a website, ect) then skip the first part and just do the copy image url thing.
I hope this made sense.... i think there is a sticky thread somewhere which explaines this already, but im bored....