The process of transferring disk images is complicated by the fact that much of the software published for the Apple II was copy protected.
Software publishers have always looked for ways to prevent people from making unauthorized copies of their software. Today, when you buy a game, it might ask you for a word from a random page of the manual, to ensure that you have purchased the game (complete with manual) and not just copied the disk. Back in the days of the Apple II, publishers were much more direct: they simply tried to make it physically impossible to copy the disk.
Unlike the PC, the Apple II had to perform much of its disk encoding in software. If programmers wanted to get tricky, they could bypass the operating system and do their own encoding, possibly changing the size of the sectors on the disk or the way in which the sectors were identified or stored. This prevented standard operating systems like DOS, along with their standard copying utilities, from accessing the disk.
However, programs which were copy protected in this manner could still be copied with more sophisticated "nibble copiers", which copied each track on the disk bit for bit, rather than copying a sector at a time. Similarly, to get a program like this to run under AppleWin, all you need to do is make a nibble image of the disk.
After nibble copiers became prevalent on the Apple, some software publishers developed tricky new ways of creating disks that even nibble copiers could not copy. It is unlikely that such a disk could be successfully transferred into a disk image.