You will have to find and use images that are more symmetrical, icons for desktop and png for objectdock are always apllied 32x32 48x48 128x128 etc. The 256x85 png you are working with, apply it as image for a dock entry and you will see it will be stretched to be a square image. It would be like, for 1024x768 screen res, using a 1024x250 image as a wall, it will fit the 1024 but will be stretched to fit 768 and look ugly.
Use images that fit icon sizing as it will not change to accomadate off size images.