First of all, you need to put the picture that you want to show on the internet, using an image hosting site such as 'Photobucket'. Once you've got the web address for the picture, click on this
link to get further details.
Where it says 'Text Body' you write the text, as normal.
Where it says 'webaddress of picture' you enter its web address (less the 'http://' of course, or else you would be repeating it).
It's best to limit picture size to about 480 pixel width or else it will appear very large in the forum (like Dustydog's).