A few ways...
If you have a photo on your computer,click on the pic with the photo&disc on, click" browse" and select photo needed and click "host it". when uploaded, copy and paste the "image" code.
If you have a photo on a host like photobucket, which is normally what i do [http://photobucket.com] then just copy and paste in the "image" code.
If there is a photo on google images you want to add, click on the normal so that you have just the photo when you like "see full size" [or whatever it says
] and then copy and paste the URL, come back here to your message and click the icon with just the photo on [tree like icon] and paste th URL into the box and click "ok".
These will all appear as codes until the message is posted, to check if you have done it correctly, click "preview" instead of "post"
Hope this helps xxx