If you allow commenting on a project Members of your Space or Guests of your project can comment. They have the option to leave a message on the entire project or annotate specific images.
To get an overview of all comments open the notifications sidebar by selecting "Comments" from the top navigation.
Comments on specific images are indicated with a small thumbnail preview of the image to the right
If you want to check the comments of a specific image, select the "comments icon" on the image card.