You can only corrupt iphoto if you try to open photos that are in the iphoto library directly into PSE without setting PSE as your external editor or exporting the photos first.
In your case, use file>Open (there's nothing to "import" in PSE 8) on the xsported images (don't drag, export from the menu in iphoto), edit and save the photos, then re-import them to iphoto as new files.
