By "checked out the photos" I assume that you mean you went into some software and tried to develop/export them? Are you shooting RAW or JPG? Are you shooting with one card or two? If two, are you seeing the issue on both cards?
I am no expert here, nor do I use a Mac, but exactly where you are seeing this matters and I assume the "file icon in Finder" is similar to the different thumbnails I see on a PC. If this is the case, that is from a separate file. Canon creates a small jpg file with every RAW/Large JPG file. My guess is that smaller JPG, used for rapid viewing, is what you are seeing as fine in the "Finder." But it is your larger RAW/JPG file that is corrupted.
This at least tells you that the issue is downstream of where the image comes off the sensor.
I would follow these steps:
- My first step would be similar to Neuro's suggestion. You may have a bad card.
- Check to see if the issue exists on your backup card (assuming you used one). If this issue was observed with both cards, I would be calling Canon as I doubt 2 cards failed at once. But it is possible.
- Check the contacts for the cards (bent pin for a CF card?).
- I might look at the contacts for the battery.
These last two are a bit of long shots, but sometimes long shots pay off. I have heard of lenses not working well and the issue being with the contact point and the solution being as simple as cleaning those points with a little alcohol.
While I have never had this issue myself, I have to think if it is not an issue with the battery, memory cards themselves, or the contacts with the memory cards, this is going into Canon as somehow something is happening internally to your camera.