Using what you have learnt about Tkinter, you will create a desktop application with a Graphical User Interface (GUI) where you can upload an image and use Python to add a watermark logo/text.

Normally, you would have to use an image editing software like Photoshop to add the watermark, but your program is going to do it automatically.

Use case: e.g you want to start posting your photos to Instagram but you want to add your website to all the photos, you can now use your software to add your website/logo automatically to any image.

A similar online service is: https://watermarkly.com/

You might need:

https://pypi.org/project/Pillow/

https://docs.python.org/3/library/tkinter.html

and some Googling.