You can apply filters to a watermark of any
kind, much as you can apply them to
the produced image.
In particular, you can:
resize watermarks using
various resampling algorithms
apply various resize
types, for example, with or without preserving aspect ratio
apply the Grayscale,
Sharpen and other filters
For example, to apply the Sharpen filter to
a watermark, go to the Watermark tab and open menu Filters > Sharpen.