I think you’re linking to the file itself, not a thumbnail URL that can be displayed as an image. I’m not sure if there’s an automatic way to do that.
I think you can add a file picker pointing to a user-specific column, then a button to “Save” that file to the database, but I don’t know how you’re structuring your data though.
I assume you have a Projects table and a Files table. A project can have many files. In that case, the button will add a new row in the Files table.
You can add a table to store Comments, and relate them back to the Project by the Project ID field. Use the same approach as the file picker, show a text entry with a button to add a comment to the Comments table, and a collection to show all added comments.