This was a great project! These are the changes I made:

I changed the background to a lighter grey, just because I like a slightly lighter canvas when I'm working in image editing software. It also looked better with the next change...

I wanted to try making one more filter, and decided it would be straightforward to make a filter on the alpha channel (data[3]). I made each click of the button subtract 51 from the alpha channel, so five clicks of the reduce opacity button will take the opacity down to 0 (assuming the alpha channel starts at 255 ... 51x5 = 255). I know only png has an alpha channel, so I'm not sure how well that filter would work with jpg downloads, but I figure that's something to play with another day.

I also changed the sepia and greyscale filters, just to play with alternate similar filters I found on stack overflow. I found some different greyscale values and a slightly more complex function for creating the sepia effect.

My code was...