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

make a temporary, backup copy of your library if you don't already have one (Control-click on the library and select Duplicate from the contextual menu) and apply the two fixes below in order as needed:

Fix #1

Launch iPhoto with the Command+Option keys held down and rebuild the library.

Since only one option can be run at a time start with Option #3, followed by #4 and then #1 as needed.

Fix #2

Using iPhoto Library Manager to Rebuild Your iPhoto Library

1 - download iPhoto Library Manager and launch.

2 - click on the Add Library button, navigate to your Home/Pictures folder and select your iPhoto Library folder.

3 - Now that the library is listed in the left hand pane of iPLM, click on your library and go to the File ➙ Rebuild Library menu option.

4 - In the next window name the new library and select the location you want it to be placed.

5 - Click on the Create button.

Note: This creates a new library based on the LIbraryData.xml file in the library and will recover Events, Albums, keywords, titles and comments. However, books, calendars, cards and slideshows will be lost. The original library will be left untouched for further attempts at fixing the problem or in case the rebuilt library is not satisfactory.

OT

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.