First, you need to stick your pictures somewhere on the internet. I use Photobucket but many consider that only really usable for light/infrequently used stuff. Each site has its own instructions on how to get photos there.
Once on another site, get the URL for the photo, that's the address starting with http and ending in .jpg or another of the image types. Then reply to a post, or create a new topic. Above the text box, you'll get the normal options, click on the add image one, I've highlighted it with a red circle in the image below:
Copy in the URL. Click OK. Then preview post to have a look and make sure it works.
Hope that helps.