To post a photo in a message you will need to have some software to edit the size of the photo and an account at a place like Photobucket. You can get the account at Photobucket free and the software many use is called Serif and is also free.
To post a photo you copy the URL for the place where it is stored and paste it into your post with [img] before the url and [/img] after it. The best size for a photo is generally less than 500 pixels wide.
When posting, you don't need to use the URL option. Only post the URL inside the [img][/img] brackets. If you edit your post and remove the part that comes before the image link it will remove that part.