Disk images originated in the 1960s, and are still valuable tools in modern macOS. They have their limitations, though, and in some cases should be replaced by APFS volumes.
DropDMG
They have more options than most new cars. What are they, how do you use them, how to pick the most appropriate, and which tools to use.
Securing sensitive data from access by others, and safeguarding it from loss of damage, is common. Solutions range from encrypted images to specialist external disks.
So many different types, from read-only disk image .dmg files, to sparse bundles and DVD/CD-R masters. Which to use, and how to create them?
Is it better to make sparse bundles ‘snug’, sizing them so that they don’t have as much free space? What if you then need to resize them?
They readily grow in size up to their maximum. But getting them to shrink when contents are reduced isn’t as easy.
The Eclectic Light Company 