Let me answer those in reverse order, if I may: Each computer has its own "bindings" in terms of what program owns what type of file. On my Mac, for example, files with the ".jpg" or ".jpeg" filename suffix are opened by GraphicConverter. Where this gets a bit tricky, though, is that Mac files also have a "creator" identification, which can indicate that a different program, say, Adobe Photoshop, created the image file.

Now we have a conflict: should the program that created the file own the file, or should your mapping of filetype to program determine which program should be launched when you double-click on the icon?

On the Mac, the creator wins out. You can test this by fiddling with the Preview program. Set your default image app to be GraphicConverter (I explain how to do that here: How to Associate a File with an Application in Mac OS X) then, within Preview, create a new image (use its slick Grab feature, for example). You'll find that the new image has a Preview icon and if you double-click it, you'll open it up in Preview, not GraphicConverter, even though you've ostensibly already made that binding.

Is that the wrong decision on the part of the operating system? I don't think so. The ideal might be for the system to ask which of the programs to use when there's more than one match, but that could get trés annoying in no time as you work. After all, if instead of double clicking on the icon you simply choose Control-click, you get a pop-up contextual menu that includes an "Open With..." that lists all possible choices:

As you can see, somehow Preview has reasserted its ownership of JPEG images on my system too (the word "default" adjacent to Preview in the menu): looks like it's time for me to reestablish the binding / pairing between GraphicConverter and my JPEG image files too, as it happens!