Nice one. I normally do jobs like this with ImageMagick but your tool work fine. As your tool runs inside the browser, as you say, the notions of "Upload" and "Download" aren't the best ones, as these suggest actions with other computers. Maybe use "Load" and "Save" ...

And as I experimented, I wanted to get different intensities of an effect to compare, but all generated images where named "noisy-image.png" ... it would be nice to have the amount generated in the names, e.g. "image-dust-30.jpg" or "image-film-45.jpg" to easily store them side by side and remember the variations applied.