Well you need to host them somewhere. If you are talking about photos of vehicles, or other things that a pertinent to this forum, you can upload them to a gallery. Otherwise, you will need to find some sort of online hosting service like www.photobucket.com
There's an icon on the 'Advanced' post option that looks like mountains with a yellow sky and a square sun (like most of my photos) If you click on it you can enter a URL (web address) of your picture.
"But what's the web address of my picture if it's in my camera (or on my computer)?" Aha, that's the trick, you have to upload you pics someplace first - THEN post them with that "insert image" icon. There's many options for where you might put your pics on the web, I see that others have covered that already.
Only thing Id like to add though, is to please remember there are users here on FTE still on dialup (like myself), and loading these pictures with that IMG inset isnt fun. A simple link to the image works just as well. Thanks