I had the same problem, but I figured out what it was. Very simple, very stupid on my part. But it works fine now. I was trying to open a PNG in Photoshop that was emailed to me. I saved it to desktop, but when I tried to open it in PS, I got that error. I was using Entourage for email. All I did was click on the security link in the email that says "click here to view the entire message" or whatever. After that, I saved the PNG to desktop and had no problem opening it up in PS. Just make sure you've downloaded the entire email message before trying to save the PNG file. Or at least that's what worked for me. Like I said, very simple, very stupid. Hope that helped.
