iMac not mounting 1990s CDs with Apple USB SuperDrive

Apple didn't introduce HFS+ (Mac OS Extended) until 1998. So if these were written before then, then they're all the original HFS, which no Mac OS after Mojave can read.

Assuming that's correct, you're going to need a Mac running Mojave or older to copy the data off so you can put it on a drive using an up-to-date disk format.

Do other CDs read correctly in this configuration? If so, that likely eliminates an adapter or drive problem.

What sort of images? Some images are volume formats not supported by macOS.

Or are these images photos? What sort of photo format was used? There were some now-gone formats used back then.

if the SuperDrive drive is working, and the volume formats is (or should be) recognized, it is distinctly possible the writable or rewritable media is no longer sufficiently reflective.

You might try a different optical drive, preferably reading at a lower or lowest speed.

Were these CD-R or CD-RW? Any information about the application or machine used to write the discs? File system details?

Have you earlier been able to read the CDs on a modern computer?

Also, one should perhaps not rule out that time has affected the discs, thus making them unreadable. Any chance of carrying out a test with an older computer/CD-ROM reader?