There is a substantial chance that nothing goes wrong with your iMessage. 1.4 Ensure that the Other Person Has Enabled iMessage Your iPhone will reboot and all network settings will be reset to factory defaults.
After that, you need to re-enter the WiFi password to connect to the WiFi network. Reset network settings will only reset the settings related to WiFi and cellular data rather than affecting other app data on your iPhone. If this problem is yet not resolved, another feasible way would be to reset network settings.
Ultimate Attempt: Update Your iPhone System.Tip: Best Tool to Fix iPhone Not Sending Picture Messages.iPhone Not Sending Picture Messages in iMessage Is iPhone Not Sending Pictures on iMessage or Regular Text Messages?.