Asssuming the pix are on your computer and you know how to resize the pix down to about 100Kbytes or smaller,
you just go to the reply screen and find "Attach File" below the reply box. It will have a Browse button over to the right. Click that and you will see your file system. Find your photo and click OK and that is that. Click "Submit Reply" and your photo will be embedded in your message.