The files you have uploaded are stored at public folder. And public folder is not deployed with code, so your files are assuming to be disappeared.
If, you use amazon S3, then images will be stored at s3 and it will provide a dynamic url to access images. Then, you will be able to access images properly.
You can also save images at dropbox. In this application images are stored at dropbox and running on heroku. You may take the referance:
https://github.com/aman199002/Album-App # open source app to store albums(at dropbox).
http://album-app.herokuapp.com #Url of the application running on heroku.