(Yes, you can store a filesystem in a file - and that's a trivial sort of disk image, but one with some serious drawbacks like "you have to allocate all of the space up front". We can do better.)
Some of the most popular disk image formats are basically a sparse file abstraction for non-sparse files and nothing more. You have a bunch of blocks, a table mapping each block to its virtual location, and a couple convenience headers.
If those count as a disk image when you put a filesystem inside, then I say a normal file is also a disk image when you put a filesystem inside.
Especially because the sparse mapping is optional. For example, lots of VHDs are a raw file plus a 512 byte footer.
