Upon opening a thumbnail to view full size image, it often spontaneously zooms to only display bottom left quarter of image.

Question

When double-clicking on the thumbnail image of an iPhoto album, the full size image will often spontaneously zoom to only display the bottom left quarter of the entire photo when it displays in full size. I can easily work around this by clicking on the right arrow to go to the next full size image (which will display properly) and then click on the left arrow to go back to the initial affected image (which will also then display properly). It does not do this when full-sizing all images from their thumbnails - probably less than 10% of the time; but still often enough to be a pian. I cannot find anything in the preferences which would appear to cause or correct it.
It is a recent problem. I have been using iPhoto extensively for many years without any problems. I cannot recall for sure, but it may have started after my update to version 9.4.2.
Using an iMac with plenty of RAM, latest version of iPhoto 11 and Mountain Lion version 10.8.2.

Answer 1

What is the format of the PS photos and are there any alpha channels in them? You might try to re-edit them with PS, making some insignificant change and then saving.  See if the new edit will display properly.  Are you saving as jpgs with the Baseline format?
I may be preaching to the choir here but this may be of interest:
Using Photoshop or Photoshop Elements as Your Editor of Choice in iPhoto.
1 - select Photoshop or Photoshop Elememts as your editor of choice in iPhoto's General Preference Section's under the "Edit photo:" menu.
2 - double click on the thumbnail in iPhoto to open it in Photoshop.  When you're finished editing click on the Save button. If you immediately get the JPEG Options window make your selection (Baseline standard seems to be the most compatible jpeg format) and click on the OK button. Your done. 
3 - however, if you get the navigation window
that indicates that  PS wants to save it as a PS formatted file.  You'll need to either select JPEG from the menu and save (top image) or click on the desktop in the Navigation window (bottom image) and save it to the desktop for importing as a new photo.
This method will let iPhoto know that the photo has been editied and will update the thumbnail file to reflect the edit..
NOTE: With Photoshop Elements  the Saving File preferences should be configured as shown:
I also suggest the Maximize PSD File Compatabilty be set to Always.  In PSE’s General preference pane set the Color Picker to Apple as shown:
Note:  to switch between iPhoto and PS or PSE as the editor of choice Control (right)-click on the thumbnail and select either Edit in iPhoto or Edit in External Editor from the contextual menu. If you use iPhoto to edit more than PSE re-select iPhoto in the iPhoto General preference pane. Then iPhoto will be the default editor and you can use the contextual menu to select PSE for your editor when desired.