Although Facebook recently passed the 500 million download mark with their semi-detached Messenger app, it looks like they're not content to rest on their laurels. After adding full Android Wear support and video uploads to the app, Facebook's latest adjustment gives users the power to edit photos before sending them to chat contacts. Well, sort of - it's the kind of editing you can do with a Polaroid photo and a Sharpie marker.
While in a chat, tap the photo icon in the bottom control row. You'll see the now-standard horizontal photo reel. Slide left or right to find the one you want, then select either the "Aa" (text) or pen icon. You can doodle or type to your heart's content, which should be very handy for elaborating on complex driving directions. Or drawing genitals on the heads of your friends when they've been caught in a compromising position. You know, important stuff. When you're done, switch down to the bottom bar again and tap the arrow icon to upload the photo.
Strangely, there doesn't seem to be any way to annotate a photo taken with Facebook Messenger's built-in "selfie cam," since those photos are sent to your contact immediately. But if you take a photo with your phone or tablet's default camera, it should show up in Facebook Messenger's photo roll quickly.