Hi Joshua, the Instagram API for 3rd parties allows only one picture to be posted at a time from a 3rd party app like Vendasta Social Marketing. I have tested this out in Hootsuite as well and it also allows only one picture at a time for Insta, as does the scheduling app Later (I use both these apps extensively for research and personal use).

If you are seeing something different in another 3rd party app could you please let me know the name of the app? Then I can track it down and see what they did.