How to prepare sensor_msgs/CompressedImage without cv2?

edit

I would like to publish a .png image so that it can be viewed on a web GUI using roslibjs. I have read that this must be of the format sensor_msgs/CompressedImage for roslibjs compatibility.

in my main.js file I have:

        let image_topic = new ROSLIB.Topic({
            ros: this.ros,
            name: '/image',
            messageType: 'sensor_msgs/CompressedImage'
        })
        image_topic.subscribe(function(message) {
            document.getElementById("image").src = "data:image/png;base64," + message.data;
            image_topic.unsubscribe();
        })

However, I am unsure how to prepare the image correctly for this in Python. I am struggling to understand what to put in msg.data. Some examples online use cv2 but I have struggled with that because other dependencies are forcing me to stay with Python 2 for now. Here's what I have so far:

    image = plt.imread('image.png')
    msg = CompressedImage()
    msg.header.stamp = rospy.Time.now()
    msg.format = "png"
    msg.data = ????????

Any ideas how to provide what msg.data needs without cv2?