This is deliberate behavior to improve the user experience on Newsfeed Facebook. Photos are automatically grouped together with a good viewing of the album, and do not make several entries in the user's news feed that may appear spam.
If you want photos to be displayed separately, you need to wait a while between messages. It is recommended that you do not send too often to the page, otherwise users may mark messages with spam, which has a negative impact on the visibility of your future messages.