If you would like to remove certain objects/defects from your pictures, try using the website cleanup.pictures.
Once you enter the site simply upload (or drag) the image and then choose the width of the brush and remove the object.
To see the original picture, press down on where it says original and you will then see a fast scroll bar.
This could be fun especially if you would like to compare one image with another with your students.
Embedded below are 2 pictures as well as a 1 minute and 31 second YouTube tutorial.