ImageJ is Java-based image analysis software that can perform a wide range of mathematical tasks on image files. It can be used to measure distances, plot brightness and color, even add and subtract images from each other. There are also a large number of programmable functions that can be defined by the user, and as open source software there are a number of developers creating new features and plugins.
ImageJ is open source software, it can be used online or anyone can download and install it for free on their personal computer.
Install Link: http://imagej.nih.gov/ij/download.html
ImageJ offers documentation and tutorials at http://imagej.nih.gov/ij/docs/index.html