- Open in Photoshop (or Gimp — the same theories apply).
- If the image is CMYK, convert to RGB.
- Size the icon to 425 pixels wide and set the Resolution to 72 pixels/inch. Make sure the checkboxes “Scale Styles”, “Constrain Proportions”, and “Resample Images” are checked:
- Save the image as a high-quality JPG at maximum quality (12).
- If the image is smaller than the target, I use a program called Perfect Resize to interpolate the pixels to improve image quality. This is the same software I use for my large format printing and it is an excellent product. It is available from OnOne Software as a standalone product as well as a Photoshop plugin.
- There is also a way to batch process images using iPhoto, but you still need to resample the image in Photoshop.
