iCloud: Comment on photos in shared albums in iCloud
You can comment on or like any photo or video in a shared album you own or subscribe to.
On a Mac using Photos
See Subscribe to albums shared by other iCloud users in Photos on Mac in the Photos User Guide.
On a Windows computer
Open File Explorer (Windows 8 or later) or Windows Explorer (Windows 7), then select iCloud Photos.
Select a photo or video in any shared album, then do any of the following:
Click again to stop liking the image.
Enter a comment, then click Post.
If you own a shared photo album, you receive a notification when a subscriber adds a comment to or likes a photo or video in your shared album. If you are a subscriber to a shared album, you receive notifications when another subscriber has commented on or liked a photo or video only if you’ve already commented on it or liked it.