You can do it via the "G Advanced" link .... or via the icons, above the Quick Reply Box where I am typing now.
The "framed tree" doesn't so much look like that as the tree is so itsybitsy! .... run your cursor over the icons and you'll find the one that says "Insert Image". Click on it and then you'll be able to post your image.
For my display it's the 4th from the right in the row of icons.