The existing "watermark" filter allows you to resize an image to be applied over another.

This resizing can be specified using the w_ratio and h_ratio parameters. Unfortunately, these parameters are only integers.
So if I want to apply a watermark of width 57px to a 600px wide image, I can't. I have to choose either 60px width (w_ratio 10%) or 54px width (w_ratio 9%).

It would be enough to allow the w_ratio and h_ratio parameters as real numbers and resizing would be exact (57px is w_ratio 9.5%).

I've already made a patch for this, which is waiting as a pull request here: #1701