There used to be only one way, upload the photo to a third party website then link to it from your post as David describes
here.
The recent changes to the forum software have now allowed us to also upload images directly. When making a post look down below the "Preview" and "Submit" buttons and notice a tab called "Upload attachment". Once you click on that you can find the photo you want by clicking "Browse...", then add a comment and click "Add the file". You should now be presented the space to add another photo if you like.