php - Fatal error: Call to undefined function: imagejpeg() on Heroku PHP. Fix?

Question

So, I am getting the following error whilst trying to upload images on my Heroku site (PHP).

 Fatal error: Call to undefined function: imagejpeg() 

It refers to the following line in my AmazonS3Handler.php file.

    //build the jpeg
    imagejpeg($destinationImage);

Any ideas on how I could fix this?