My otree experiment uses the Pillow package. I tired to deploy my experiment through otreehub, but I get an error. Pillow is added to my requirements.txt, but I still get the following error message when I reset the database:
 ModuleNotFoundError: No module named 'PIL'
 Process exited with status 1
 
 I guess the problem is that I should install Pillow on Heroku, but I am not sure how to do this. How can I install a package on Heroku?  Could yu please give me a step-by-step guide?

Double check your requirements.txt. Note the comment at the top about otree automatically modifying that file.

Is it a problem if I let otree automatically modify the file?
What is the recommended way?
# oTree-may-overwrite-this-file vs. # oTree-may-not-overwrite-this-file?

I realized that Pillow vanishes from my requirements.txt again and again.

It seems that I could solve the deployment issue itself. However, the experiment creates images with Pillow and those are not displayed on heroku, although importing Pillow has been accepted in the deployment process and images worked on my desktop. Do you may be have experience with this kind of issue?