Thank you for you fast answer.
-Are you converting multiple images at the same time?
No we are converting one a time.
-Have you measured which part of your action is slow?
Well the think it is consuming 100% of the CPU. So take make it slowly. I dont now exactly what is causing it.
-Is it reading/writing of the image or is it the resize action?
if (img.Height > img.Width)
img.Composite(wtmark, Gravity.Southeast, CompositeOperator.Atop);
-And what do you consider slow?
So it is slow when the cpu get the 100%.