This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
First, to the best of my knowledge, only versions 6.x and 7.x of Copy II Plus can create a disk image file (called an "IMG" file). The feature was gone by version 8, for sure. To create an IMG file you COPY--> DISK to an over-size target volume. Versions 6 and 7 will create a type "IMG" file instead of complaining about a "size mismatch" (which is what other versions of Copy II Plus do). It is fairly common to end the name of the new IMG file with ".img". Doing the opposite lets you convert an IMG file back to diskette. That is, you select the COPY --> DISK option and pick the large volume with the IMG file as Source and a blank unformatted 5.25" diskette in the Slot 6, Drive 1 drive as Destination. You pick an IMG file on the Source volume and it is transferred to the diskette. Copy II Plus IMG files are not compatible with DSK2FILE or ASIMOV and will not work on emulators such as AppleWin. Also, not all files ending with ".img" are Copy II Plus disk image files. I have seen ".img" (and ".image") used for Diskcopy disk image files. By: Rubywand