When you're typing down in the little reply box at the bottom of your screen, there's a series of icons across the top of the window that you're typing into. The one with a little up-arrow (it's right about smack in the middle of the icons) is an Upload icon (if you hover your mouse over it, it'll say "Upload". That's the only way I found it).
Click on that icon, then it'll take you to a window that says
"From my device... [choose files] no files chosen"
If you click on the "choose files", it'll open up a window where you can click on a photo image that you've already saved to your computer.