Because the image is fixed in size, you can use an image that is twice as big. So for example, in a 128x128 container, you can use a 256x256 image, but resized to 128x128 pixels.
Because the image is fixed in size, you can use an image that is four times as big. So for example, in a 128x128 container, you can use a 256x256 image, but resized to 128x128 pixels.