Cant really help but the best advice i can give you is not to download Dreamcasts images other than Discjuggler/.cdi files.
Lots of people who put there releases on P2P things just make a image of the disc they had with Nero or Clone cd(
) and expect them to work without testing them before they share.
Dreamcast games were released as cdi images so its best to stick with that
Having said that tho have had a few nero images and they worked fine but its best to stick with discjuggler images