I have used all three of the tools mentioned for Macs.
I installed
Graphic Converter for my parents, on a OS 9 machine which had no other suitable tool; it's fine but I think most OS X users would be more comfortable with something that has a look and feel more consistent with the
iLife applications (which are generally quite functional) for simple tasks like preparing photos for FiberglassRV posting. For
Windows users,
Irfanview is somewhat similar to Graphic Converter, and I think it is very good; it is what I use for my posted images.
When my parents went to OS X, they switched to
iPhoto, which I believe was appropriate for their purposes. It is primarily an album tool, rather than a photo file manipulation tool like Graphic Converter. I found the export feature less than entirely obvious on my first try, but ended up (I think) with the method which Mike provided; it works fine for the purpose of sizing a photo for posting.
I don't think the iLife stuff is actually free - it's just that you don't buy a Mac OS without buying the computer, and iLife comes bundled with it. Patches are free but major releases (which seem to occur annually) have a cost. When you buy an OS upgrade (another almost annual experience), you may or may not get an iLife upgrade with it.
The full
Photoshop product is the gold standard in photo editing, and is massive overkill for most people. I have paid the several hundred dollars for a copy which we run on a Mac. Apple is now promoting their
Aperature as an alternative product, at a similar price; it is part of their Pro lineup, and not intended to replace iPhoto. Most people find Photoshop too difficult to use on a casual basis, because there are so many features that the interface is necessarily complex. I wouldn't suggest to anyone that they use this class of product unless they are seriously interested in imaging.
A few years ago, Adobe effectively replaced the old PhotoDeluxe and Photoshop LE with
Photoshop Elements; there are now "Elements" versions of a few Adobe products. Photoshop Elements uses much of the core of the real Photoshop, and is a surprisingly powerful tool for the price. I have only used the PC version, but there is a
Mac version, and if I had to start from scratch again with a Mac I would seriously look into this for more advanced photo manipulation than iPhoto.