I have seen sometimes in my own tenant that this change can take 2-3 days to reflect.
Here are some steps to try:
- Login to Azure Portal using a user whose picture needs to be changed
- Switch the directory of the user to the default directory through Azure Portal
- Go to Azure Active Directory, All Users
- Select the user, in the profile change the photo
I tested in my tenant and it worked.
It can help to try in a brand new browser with a cleared cache and incognito window. Occasionally I have had to do this.