Several, completely different types of ISF files exist, but only one of them is used to store graphics content, the diagrams from a tool called Inspiration. And it is actually only this software that can be used to export the diagrams to standard graphics formats, including JPEG or PNG formats.
Unfortunately, we do not know about any alternatives to his as this type of .isf file doesn't seem to be supported in any other software.
Updated: September 25, 2023