isobuster can convert disc images directly without mounting anything and also has a free use version, there is even a guide for that
here and works fine for PSX games that other direct converters fail with and works fine for DVD images aswell with the exception of GI DVD images that hardly anything supports (D-Tools does not support those either) but those are not common either.
The only disc images I would not touch in isobuster is Dreamcast image but you should never try and convert those unless you know what you are doing.