Hello Kibble_Eater,
Thanks for posting in the Apple Support Communities.
This sounds like you can send and receive group iMessages, but can't send a group SMS message or photo, correct?
If we have that correct, let's first be sure you have the latest iOS version installed on your iPhone to see if that could clear this up. If an update is available, back up your iPhone before getting started with the installation. For assistance with backing up or updating, check out this resource: Update your iPhone or iPad - Apple Support
Once you've confirmed you have the latest iOS version installed, let's give the steps below a try to see if that helps. If you still can't send the messages, contact your carrier to see if a change needs to be made for MMS Messaging on their end. You can also review additional steps to try here: If you can't send or receive messages on your iPhone or iPad - Apple Support
If you still need help
• Try to restart your iPhone or iPad.
• Check your network connection. To send a message as iMessage or MMS, you need a cellular data or Wi-Fi connection. To send an SMS message, you need cellular network connection. If you turn on Wi-Fi Calling, you can send SMS messages over Wi-Fi.
• Check with your carrier to see if the type of message you’re trying to send, like MMS or SMS, is supported.
• If you’re trying to send group MMS messages on an iPhone, go to Settings > Messages and turn on MMS Messaging. If you don't see an option to turn on MMS Messaging or Group Messaging on your iPhone, then your carrier might not support this feature.
• Make sure that your device has enough space to receive images and videos.
• Make sure that you entered the correct phone number or email address for the contact.
Let us know if you still have a problem after working through those steps, and contacting your carrier.
Have a great day!
