Here is a little picture tutorial.
To add a picture, start by hitting Post Reply or Go Advanced.
Hit the paperclip icon to open an attachment upload window. Notice the list of valid file types and the limits to the file sizes. Simply browse for the files you wish to add and then press Upload.
In the document they are added to the bottom by default but if you prefer to control the placement, set the cursor and then hit the down-arrow just right of the paperclip and choose the attachment you want in this position.
That is how you add an attachment to your forum post.