This is misunderstanding of the definition of web server — it should only be used to “talk” with clients. Heavy load tasks should be delegated to standalone programs (that of course can be also written in JS).
You’d probably say that it is dirty, but I assure you that a web server process stuck in resizing images is just worse (even for lets say Apache, when it does not block other queries). Still, you may use a common library to avoid code redundancy.
EDIT: I have come up with an analogy; web application should be as a restaurant. You have waiters (web server) and cooks (workers). Waiters are in contact with clients and do simple tasks like providing menu or explaining if some dish is vegetarian. On the other hand they delegate harder tasks to the kitchen. Because waiters are doing only simple things they respond quick, and cooks can concentrate on their job.
Node.js here would be a single but very talented waiter that can process many requests at a time, and Apache would be a gang of dumb waiters that just process one request each. If this one Node.js waiter would begin to cook, it would be an immediate catastrophe. Still, cooking could also exhaust even a large supply of Apache waiters, not mentioning the chaos in the kitchen and the progressive decrease of responsitivity.