Can java write code for a web page that can manipulate images?

[Chris]

Javascript can turn an image into an array of pixels and vice-versa, so you can use it to apply filters. I don't think you can save the result though.
PHP can also manipulate images, I have written code that turns a photograph into a three color image (posterization).
In general though (and I assume Instagram does this), the uploaded image is saved, then php (or whatever they use) calls an executable that will process the file.