The icons need to be the exact size asked for, I use Fireworks and usually make the largest size icon required (1024×1024) with resolution at 75pixels/inch, then when I am happy with it I save as an editable 32 bit PNG file. I then export the image as a 24bit PNG and close the file. You can then open the exported 24 bit image and resize to 512×512, then export and save as the new size. Then do the 120×120, 96×96, 60×60 and so on till all the images are sized correctly.