Heroku Nodejs get image 404

I'm using MEAN Stack(express and nodejs) to deploy my project into heroku. I'm using a sandbox database provided by mongolab(mlab) addons. My question is why does the the image file cannot be displayed after a certain period of time(after 30-40 minutes)? The system has no problem when reloading. The image will still be there. But after the period mentioned, the server cannot Get the image and return 404 error. Here's an example: enter image description here How to solve this?

via Azinuddin AleleCha