I am doing this by memory so it may be a bit inaccurate.
Theres a few tabs at the top, lcd and logo-fb, click logo-fb and then load, then choose the image you want to convert, it may say it needs resized so click YEs, if it then asks if you want a border around it click No.
I think you have to then convert it, may be 3rd button down but not sure, it will save the file with the extension .M1V.
You then go to where you saved it and rename it to logo-fb (no extension) and ftp it to the dbox. /var/tuxbox/boot
I think you can set up to network it across with logo viewer but I never bothered and just did it myself.