If you're using Apples photo app, you must first export the pictures to your desktop. Doing so will put them in folder(s) on the desktop. From this forum, go to advanced tab and attach your photos.
Btw, at times the picture may appear sideways or upside down. If so, open the photo on the Mac by double clicking on it. It will open in prewiew. Rotate the picture to correct it and reattach.
Hope that helps,