We use Fenix to build Ubuntu/Debian images, The default logo is archives/logo/bmp/logo.bmp, so you can use your own bmp file to replace the default one.
Note: The BMP file should be 24 bpp.
You can also to replace the logo file directly in the ready to use image. The logo file is in /usr/share/fenix/logo/logo.bmp, so you can replace this file directly.