I am creating a mongodb/nodejs blogging system (similar to wordpress).
I currently have the images being saved on the disk and a pointer being placed in mongo. I was
Storing the image as document in mongodb would be a bad idea, as the resources which could have been used to send a large amount of informational data would be used for sending files.
Have a look at mongoDb file storage GridFS , that might solve your problem of storing images, and providing horizontal scalability as well.
http://www.mongodb.org/display/DOCS/GridFS http://www.mongodb.org/display/DOCS/GridFS+Specification