1. Go to "Image" and click on "Resize image"
2. Change the resolution of your bootscreen from 640x480 to 320x400 by using the "Bicubic" setting.
3. Save it as a PNG image with the 8-bit depth setting.
4. Then, go to "Image" again and click again on "Resize image"
5. Change the resolution of your bootscreen from 320x400 to 640x400 using the setting "Nearest Neightbor"
6. DONE! You must have a realist 9x-looking bootscreen.