Welcome to the complete BS nonstandard image/disk format of the Macintosh. Seemingly doing everything possible to make something like disk images impossible for people not in the know.
This is a bit unfair. Disk images (and the Disk Copy application) have been apart of the Macintosh ethos since its inception in 1984, with the Disk Copy 4 format released only a few years later. DC4 and later DC6 image formats haven't changed in nearly 40 years, and a lot of the software now available on places like Macintosh Garden were actually imaged 20-40 years ago; said software had previously been floating around on usergroups, BBSes, Gopher, FTP, and early internet archives for
several decades prior to being uploaded to places like Macintosh Garden. Only relatively recently have they been uploaded to more modern websites as trademarks expire and copyright exemptions are made for vintage software. So to say that it's nonstandard bullshit is not accurate.
The problem was never the Macintosh, just that users new to vintage Macs are simply unaware just how long software archiving has been going on with the Macintosh in general (which is to say,
the entire existence of the product line.) A
lot of the software currently on the internet has remain unchanged since the 1980s and 1990s.
However, it would be nice if there was a concerted effort to convert all the original software to more portable formats like raw DSK and ISO files.
For now, a general rule of thumb: Use the Disk Copy 4 format for floppy images. They don't need to be encoded, and can be safely shared with non-Macintosh computers. The format is widely supported by both software and hardware emulators. You can either use the Disk Copy 4.2 application (which I recommend for floppy disks), or Disk Copy 6. Just make sure that if you use Disk Copy 6 to save it as Disk Copy 4 format.
CD, DVDs, etc. use common sense and just save as an ISO.
If there ever is a major shift in all the software archives out there, my vote is for the .MOOF format. It is specifically designed for Macintosh disks. It is open, documented, and has support for copy protection built into the format. This would enable previously-uncopyable software to be made available in an easy-to-use and portable format. More convenient than a raw flux. It's already being supported by several floppy emulators.