Convert a plain disk "legacy image" so that it can be mounted
This worked on High Sierra. This is from an ancient DC42 image format from a 512Ke system disk.

Todds-MBP-7:$ hdiutil convert Utilities1.image -format UDRO -o Converted
Preparing imaging engine…
Reading whole disk (Apple_HFS : 0)
.........................................................
(CRC32 $7335686D: whole disk (Apple_HFS : 0))
Adding resources
Elapsed Time:   2.822ms
File size: 729282 bytes, Checksum: CRC32 $E4490B5F
Sectors processed: 1600, 1417 copied
Speed: 245.2Mbytes/sec
Savings: 11.0%
created: /Users/WFN32919/Downloads/converted.dmg