It is not suggested to store images in a database, because you have performance issues when pulling long strings... The suggested solution is something like s3 from Amazon which stores the actual files, but since your question is how to store an image in mongodb here is what you need to know.
This is stored as a dataUrl aka a freakishly long string not entirely ideal for sending in Json responses. You will need to convert the image to a base64 representation of the image and here is a link to an explanation of your stored string format.
How to implement -toDataURL() function in node.js server?
Here is a link on converting an image to base64 using node.js
NodeJS base64 image encoding/decoding not quite working
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