In the reply window, click the 5th button over from the left, looks like a brown house with a blue arrow. Then follow these steps:
1) Click the "Browse" button and point to the image on your drive. Accepts either jpg or png format.
2) Once selected, Click "Add this File".
3) Repeat steps 1 and 2 if you want to add more than one image.
4) After you're done adding images, [ Save all images ]