First, you'll need an image. The standard console is generally 640x440, but reducing itd height is a good idea, because the prompt will appear under the image, or it will push it out of the screen if the image it too high, ot if you input too many lines. So, a good idea would to use an image size between 640x320 and 640x400, depending on the amount of infos you want to display.

Next, you must reduce your image's palette to at most 16 colors, for example with the Gimp, and choose your prompt's colors as well : if your image has no white neither black, but you want a white on black prompt, you must reduce your palette not to 16 colors, but 14 (the prompt colors are selected from the image's palette at boot time).

If your prompt foreground and/or background are inside the palette, please write the colors values in hex form somewhere, because you'll need to type them in the command line.

Then, export the image as a ppm file.

To convert the image ot a lss file (which can be read by isolinux) you must use the ppmtolss16 tool :

  ppmtolss16 "#000000=0" "#ffffff=7" < image.ppm > image.lss

In the lss file, index 7 of palette will be used for prompt foreground color, and index 0 will be used for its background (here, it's white on black, but you can replace them with your own colors of choice).

If the colors you define as palette index 0 and 7 are already in the image's palette, it will only be reorganized. In the other hand, if the colors are not already used in the image, they will be added to the palette, and then, if the palette is more than 16 colors, it will be reduced to this number, which can cause your image to be uglyly displayed in the end.

That's why you should think about the prompt colors when reducing the original image's palette (to 16, 15 or 14 colors, if you use 0, 1 or 2 additional colors for prompt).