I wrote an app to handle group texting ("TextShot") so I do know a bit about this.
Go to the Messages page of Settings, and if you see an option for "Group Messaging", turn that off.
Group messaging means the group is linked, so any reply to your text is like "reply all" in email and the entire group receives any replies to the original message. The linked group size is therefore limited to 10 contacts to stop this all getting out of hand.
Turning group messaging off will allow you to now send MMS messages (i.e. with photos) as separate messages to more than 10 people at once.
If you don't have the "Group Messaging" option (it depends on your carrier) you would need to turn MMS off entirely to send messages to groups larger than 10. This will mean you can only send SMS, and can't include photos.
I hope this helps.
Peter
Soluble Apps