On a computer, click on Go Advanced below the Quick Reply window.
Then click on the little paperclip in the top row. It will open up a new tab. Make sure you turn pop-off blocker off or allow pop-ups.
Click Browse, and navigate to the photo, then click Upload.
Once the upload is done, then go back to the original tab that has the reply window, click the drop-down arrow next to the paperclip, then click on the photo name and it will insert it into your post.
Let me know if you are using a phone or tablet, or an app like Tapatalk.
-Dan