Do you mean how to add an Avatar photo to your profile?
Create a photo or image at least 400x400 pixels
Log in if needed
Click on your profile name near upper right of screen
Click on Account Details
Click on Avatar
Set other options there as desired.
Or do you mean how to add photos to a message?
Click on the little photo rectangle in the formating bar at the top of a message. Select image
Or, locate a photo in the OS file system, copy, paste into message. Click on a corner and drag with mouse to make it a reasonable size. Can add up to 4.
Or, click on the "Attach files" button at bottom while composing a message.
Select a photo file. Click on the Insert button in the thumbnail and select thumbnail or better, full image. The resize as above. Repeat.