Upper top right to the right of red subscribe box - 3 icons over is 'my profile' icon, menu drop down > My Image Gallery and on that page is the old way of doing it as explained left side.
Newer way as P&V said, save a file to the desktop, then open the message field as if to type a new post and drag the file into that field.
It now automatically goes into your gallery library while posting real time to the thread after you hit submit.
In many cases you can also drag an image from another tab or website as well. Some sites don't allow that.
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