Upon opening a thumbnail to view full size image, it often spontaneously zooms to only display bottom left quarter of image.
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.
iMac, OS X Mountain Lion (10.8.2)