Some iPhone users who updated to iOS 11 can no longer see the "Message" icon in the share sheet when in apps such as Photos, which means they can only text or iMessage an image from within the Messages app itself now. This is a bug with remote management software which will hopefully get fixed soon, but until then, the solution isn't pretty.
Why isn't it pretty? Because you have to delete the remote management profile off of your iPhone in order to do it. That means deleting apps such as MobileIron and VMware AirWatch along with any profiles visible in "General" from the Settings app.
For some of you who are using business iPhones, removing this software is not something you can do, so you'll just have to wait it out until a patch hits your device, I'm sorry to say.
If you've found another solution, make sure to let us know in the comments below!