Welcome,
Finding particular posts can be difficult, I couldn't find a step-by-step tutorial there myself. I'm not an expert, but I think I can help you.
You do need to upload your photos to a third-party site. I lot of people use Photobucket. I use Windows Live. I suspect Flicker would work.
Once your photos are there, you have to find the specific URL that just displays your photo. I simply keep clicking on my photo until it is the only thing showing on the screen, then I copy the URL out of the address bar.
Back here on FSM, there is a little icon that looks like a strip of film:
Click that and paste the URL into the field for it and click insert.
Tahdah, you have posted a picture!
Paul