My bad, I thought it came with all OS X packages, but it looks like it was a hardware-specific bundle:
http://lowendmac.com/designer/07.html
So if the OP has one of these machines, they have it.
I have an iBook G4, and don’t have it in the applications directory. I guess the loaded software on this machine is a bit dated.
There’s nothing in the OP that says to me that a fully featured bitmap editor is needed. As long as the edits are fairly simple, Graphic Converter is a better choice due to the ease of installation alone. I’m not knocking GIMP, I use it in Linux and Windows. But I rarely fire it up for simple edits that don’t require the additional functionality if something like Paint.NET can do the job.
In my opinion, Graphic Converter is better at being Graphic Converter than GIMP is at being Photoshop. I realize that I’m holding them to different standards, but the way that the apps are promoted pretty much means stacking the deck that way.
To elaborate on this, a .app on a Mac isn’t a normal file-- It’s actually a folder, with the actual executable somewhere inside of it, along with the various other files it needs in order to run. You can see the innards by right-clicking the file and selecting “Show Package Contents”. The same is also true, by the way, of .rtfd files (rich text with attachments, such as images).
Back to the OP, personally I use GraphicConverter. It’s been able to do all of the things I’ve asked of it, though it’s admittedly sometimes difficult to figure out how: Their business model seems to be to give the fully-functional software away for free, but to charge for the documentation. It came pre-installed on one of my machines, but not the other.