There are many reasons why photos are not showing up in Messenger. But in this I am going to tell you some specific reasons and important reasons.
Update the Messenger app
Go to play store or app store and update the messenger app.
Clear the Messenger app cache and data
Go to settings and clear data of messenger app.
Restart your device
Try restarting the mobile.
Photo permission
Go to mobile settings and click on messenger application. After that you will see the messenger. Go to Permissions by clicking Messenger and OK all permissions.
How to do it in iPhone.
Go to iPhone Settings and click on Privacy then click on Photos.
Temporary server issue
Sometimes photos are not sent due to server issue of Facebook Messenger.
Reinstall the App
Reinstall Facebook Messenger
Update Operating System
If there is a software update in the mobile, complete it.
Internet Connection
Messenger can't see photos because of slow internet connection.
Don't use a VPN
If the problem persists, contact Messenger
Always download facebook messenger from google play store or app store.