To upload a picture to the forum or include it in a post, please add it to the Uploads folder of the Photos section.
( Go to the Photos section of the page, chose the the Uploads folder, click Add a picture, ( end up here: http://lavadomefive.com/cs/controlpanel/photos/posteditor.aspx?sectionid=23 ) then click Upload Photo, add whatever tags and info you like, preview if you like, and then hit Save. )
If you want to add the picture in a post you can either use the Content Selector (between the little tree and the smily in the post editor icon bar) and select it from the photos section there, or go to it in the Photos section, copy the adress of it, click the little tree instead and paste the adress.
Of course you can just add it in plain html code too if you prefer, now it is uploaded and has an adress. 
If you want to upload your art / want a gallery you can of course get your own Photos folder for that!
Just ask the Dinosorceror!