Unable to post photos in text messages on iPhone
Why can’t I post pictures on my iPhone in a text?
[Re-Titled by Moderator]
If your iPhone does not have a suitable Wi-Fi, or cellular data connection, then the only way that it can send a photo is if you have MMS enabled in settings. Ideally, if you have a cellular data connection or are on Wi-Fi, your iPhone will send iMessages to other iPhone users, or RCS messages to Android users. Some information about the differeences is listed here: What is the difference between iMessage, RCS, and SMS/MMS? - Apple Support.
To enable MMS to send photos using SMS/MMS technology:
Settings > Apps > Messages > Send as Text Message > ENABLED
Settings > Apps > Messages > MMS Messaging > ENABLED
Also make sure that RCS is enabled:
Settings > Apps > Messages > RCS Messaging > RCS Messaging > ENABLED
If this still does not allow sending photos, then contact your cellular provider.