You have to understand that a filename (and/or an extension) means NOTHING unless a given convention is in use.
Naming a floppy image with a .ima extension (right[*]) or with a .img extension (wrong[*]) or with a .imgPTN (Steve6375's "own" extension used for Easy2boot[*], that actually is normally applied NOT to a floppy image, but rather to a ParTitioN image, i.e. a particular form of "superfloppy") does not alter in any way the actual contents or the "essence" of the file at hand.
And there are a number of confusing terms in use in technical literature/knowledge (on which a few people, including Olof, Steve, erwan.l, dencorso, a few other members and of course yours truly) debate since years , like device/disk/disc/drive/partition/volume/floppy/superfloppy/filesystem that are often used - if not improperly - loosely and that need to be attentively evaluated and understood in the context they are cited.
A "floppy" image is a volume with size/characteristics compatible with "existing" real hardware floppy disks. <- this is the image of a non partitioned "real" device with Sectors Before=0 in the BPB (and should have IMHO .ima extension for historical reasons, as the extensions was used by the early programs such as Winimage since the dawn of time)
A "superfloppy" image is a volume as the above but usually larger and either corresponding (as an example LS-120 or ZIP disk image are "superfloppy" images corresponding to "real" superfloppy devices) or not corresponding to real hardware (as an example the 5760 Kb floppy) <- since these are still - essentially - (bigger) images of non partitioned devices (Sectors Before=0 in the BPB) they should also have the .ima extension
A "hard disk image" is the image of a partitioned device containing one or more volumes <- this - traditionally - had the .img extension.
A number of programs/tools use for the above indifferently the .img extension which helps in the confusion[*].
A "partition image" (as Steve6375 defined it) is actually an image of a primary partition, i.e. the image of a volume on a hard disk, conceptually it is also a "superfloppy" BUT it is actually IMHO a very good thing that he found a suitable extension to "identify" and distinguish this type of images, which - by definition - have NOT Sectors Before=0 in the BPB (at least when "applied")
As well as the n extensions he "invented" are a good thing :
because they provide an easy way to know "which is which" by the extension, AND provided a documented convention, but you cannot expect any other tool (or Author) to follow that same (partially arbitrary) convention.
[*] "right" or "wrong" only depends on the conventions used or not used, it is common to find more extensions used for floppy images.