Hi Brownbutterfly,
Welcome to the Apple Community.
Wondering if you are trying to send the messages over WiFi with the Messages app.
How to: Send and receive photos, videos, and audio messages on iPhone – Apple Support (AU)
Else :
If you use SMS for texts. and MMS for texts with photos etc., on iPhone, you need a text messaging plan.
You might need to check with your wireless carrier whether you can send MMS on your current plan.
Apple Support's article: About iMessage and SMS/MMS on iPhone: may help make things clearer:
What is the difference between iMessage and SMS/MMS? – Apple Support (AU)
Hope this helps :-)