Are free operating system ISO images legal?

Yes. ISO images for free software operating systems like Linux, FreeBSD, ReactOS, or OpenSolaris are usually legal. However, on the off chance that it contains material that is not legally redistributable, such as unlicensed music or video, then that particular image would not be legal. In some areas, software like libdvdcss is also illegal, meaning if it is not legal in your country, then you should not download that ISO image.