Hey Marc,
Below the "Reply to Thread" box (the box in which I am now typing my reply)
and just below the heading icon selection, there is a bar marked "Additional
Options."
If you don't see the box underneath the bar, but just see the bar, there is
an arrow at the far right of the bar that toggles the box on and off.
In that box there's a "Manage Attachments" button, which opens a window.
In that window, "Browse" your PC for the file (must be a listed type), select it,
then click "Upload". The image will NOT show when you preview the post but
you will see the filename and size above the "Manage Attachments" button.
The image will show in the thread after you "Submit Reply."