Size and Scaling of Watermarks

I found many requests about the scaling of watermarks, but I did't find an acceptable answer. Is it possible to use different watermark images for the different sizes of a picture or is it only possible to use the same watermark for all states of an image?

so is my 120x60px watermark on pictures with 640x480 and on original size and on the thumbbnail and in all three versions always with 120x60? Or will it be scaled with the image, or can i define different watermarks for the different stages of an image?