Hi ​@JenaM88  is it just pictures via cellular that are the issue, are picture messages coming through via RCS from the same contacts?

No nothing comes through in cellular or RCS. And group messages with some iPhone users as well don't work. But others work

There are some eligibility conditions for all parties as group messages whether delivered in text or picture are recognized in our systems as MMS. MMS capability must be enabled for all users for group messaging to work. All users' devices must have a group message compatible device and the option turned on. 

If this is only happening with some iPhone users and not others or only some of the time, it’s likely some compatibility issue on the other end. You can also check if it’s hardware related on your end by testing your SIM card in a different phone. 

But it's not just group messaging. It's also sending or receiving pictures from those people.

iPhone users normally iMessages to send/receive photos. When sending in the group, iMessage message might not be converted into MMS and you couldn’t receive it. I would recommend you ask those specific iPhone users in your group to check to ensure they sent it out in MMS format.