You should contact Avast support as we can't help you with their products.
IMO, if trojan is inside ISO then it is harmless. When you will extract it and run it then it can be a threat. While extracting ISO Real Time Protection of Avast should detect it and take necessary action against it. You should contact ISO's source and ask for uninfected ISO.
Another chance may be that that ISO is not infected, avast is showing false positive, but you should not take chances.