CamanJS is a nice little library for manipulating your images with Javascript:

CamanJS is (ca)nvas (man)ipulation in Javascript. It’s a combination of a simple-to-use interface with advanced and efficient image/canvas editing techniques.CamanJS is very easy to extend with new filters and plugins, and it comes with a wide array of image editing functionality, which continues to grow. It’s completely library independent and works both in NodeJS and the browser.

Source: CamanJS – Javascript Image Manipulation