Inspiration diagrams (.isf) can be exported and saved in all kinds of graphics files including PNG format. However, this seems to be only possible in the original software and no other. There seems to be a free trial available on the developers website.
Convert Inspiration diagram to Portable Network Graphic.
We found one software available in the database for this conversion.
Microsoft Windows software - convert isf to png on Windows
A ultimate brainstorming software tool
Convert from isf file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.