PB 602/603 needs a BMP image, 600x800 pixels, grayscale, 96dpi. You can use Irfanview in Windows (
www.irfanview.com) to modify whatever you want, or GIMP in Linux/Windows. Rename your image according to your existing logo placed in your pocketbook memory, in directory /system/logo (I have a 603 and mine is named "1_st.603.bmp") and you've done.