Grunt's got a good plan. The key thing you'll need to decide is which app/method you will use to get stuff off of your S3 and onto the iMac. AirDroid, as noted earlier, is quite easy to use and I've had no issues with it via Safari. I was also using Kies Air on the S3, but the latest update (last I checked in early Nov), was badly broken so I removed it. It's unfortunate as it was functional and "native" to the phone. Check out the reviews in Google Play store and you'll see why it makes sense to avoid it for the time being. AirDroid is a good alternative, and there are other quality tools similar to this in the Play store.
Once you're connected to your phone in the iMac AirDroid browser session, you can navigate to wherever you are keeping your photos on your phone. Then using Grunt's plan, get your stuff into iPhoto. I personally use the Pictures in Finder, and within Pictures have all of my photos organized in subfolders based upon topics like vacations, kids, etc. The key step and concept is to get your photos copied over to your iMac (either using AirDroid or Kies, etc.), and then you can copy/import stuff into iPhoto.
Lastly, don't think you're being daft as the current state of media synching to/from Android to Mac ecosystems are not as intuitive and straightforward as it could be. Be patient, though, as there are plenty of good options and tools out there not only for photos, but music, etc. When your company does make it big, certainly keep us in mind for that CTO spot
