Tiff files that I created with the System, i.e. screenshots, edit OK in iPhoto and Photoshop CS3. The jpg file that's created from those tiff files are readable by PS.
However, tif file created by PS and imported into iPhoto don't get converted to a usable jpg file.
What I think is happening is iPhoto is changing the extension on the tif file to .jpg without changing the actual format. I read one of the jpgs that iPhoto tried to create as part of the editing with a couple of EXIF reader apps and one showed the file to still be a tif and the other reported "No or invalid EXIF header"
To confirm that I took a tif file and changed the extension to .jpg. When opening with PS I got the "No or invalid EXIF header" message.
I'm betting that iPhoto is doing the same with png files, changing the extensions without actually changing the format.