So, to be explicit:
1. Hit Reply
2. Type a message
3. At the image upload section, click "browse"
4. When the dialogue box opens, find the file on your computer
5. Select the image file and click "open" (or a double-click on the file does the same thing)
6. This returns you to the PAFF form and you should see the path in the file location area of the form
7. Now, hit the Upload button to the right of the Browse button
8. After the file uploads, the image name will appear near the bottom of the reply form*
9. If it doesn't, something was wrong with the image parameters
10. Finally, click Submit to post.
*See photo below