Why is the product photo not appearing on Facebook ?

If you are on PrestaShop 1.5 / 1.6, make sure you first also read our FAQ on delayed posts

Also, please make sure you have an adequate image size selected in the module's configuration in the "Module options" tab. Usually, the bigger the better, so thickbox or anything from 600 x 600 to 1000 x 1000 is good.

You'll also want to make sure that your miniatures have been properly generated if you just created a new image size (Go to "Preferences" -> "Images" in your back-office).