Enable iMessage

Have iPhone 6+ with IOS 8.1 installed. When texting, I attempted to send a message with 3 photos attached and received notice, " Cannot Send Message. iMessage needs to be enabled to send this message. Cane send the message with no pix attached. The addressees are mixture of Apple and other users. Under Settings>Messages, iMessage, Send as SMS, MMS Messaging and Groups Messaging are all chosen. Any ideas?

If you can send the message(s) without attachments but not when you attach the 3 photos, I wonder if it's some kind of size restriction by your carrier? All your settings on the iPhone are correct.