If it is a photo on your computer and the photo is small (<64kilobyte) you can upload it to this website and attach it to the post (click the "upload attachment" tab below the text entry box and navigate to your file).
If the photo is more than 64kB, you'll have to find a way to host the image externally from a different website.
You can link to an externally hosted photo using the image tags "
[img][/img.]" (highlight the link then press the "Img" button).
The code to post the photo
http://www.safespeed.org.uk/ss150.gif would look like
Code:
[img]http://www.safespeed.org.uk/ss150.gif[/img]
That code results with: