After placing the image icon, open the control and it will ask the image. Give the image path and then in the below we have an option called the custom. In the custom you can give the size of the image which you want.
How did you do the header resizing on the skin rule? I'm trying to do the same thing as the png image I added to the Portal header group primary in pyPortalHeader is not getting the image size correctly, it is too small.