720 x 576. There is no such thing as DPI in this context. The image has to be encoded to strict standards, or the box will crash when trying to boot. There is a TMPGenc template, but it's probably best to use one of the apps designed specifically for this purpose.
Either - it doesnt matter. Just run the image file through LogoViewer and save the file as a .M1V format. I usually save it to the desktop. Once saved you can then preview the still logo in Windows Media Player.
Upload the file to the box and rename it to LOGO-FB (lowercase) and WITHOUT the .M1V extension.