Why does a black triangle appear instead of landscape photos from a shared library

Question

We have an iMac using OSX10.7.5 and running iPhoto 11 v9.4.1. 
There are 2 users of the iMac with separate logins.  We have an iPhoto library that is user 1 administers and imports photos to, edits photos etc.  User 1 can access and see all photos without any problems in thumbnail or large scale/full screen under their login. 
When user 2 accesses the photos on the user 2 login, all photos can be seen in thumbnail, but when double clicked to show photos at larger to full screen size, all portrait pictures show ok, but after a first landscape photo sometimes showing, the rest of the landscope photos produce (sometimes) a flash of the picture but then the area of the photo goes black with a warning sign, that is a triangle with an explanation mark appears in the centre of the screen.
If user 2 runs a slideshow, the portrait pictures show up, the screen is black/blank whilst landcsape photos would be shown
A very few landscape photos can be seen and they all appear to be files of less than 1.5mb, but this does not apply to all photos of less than 1.5mb.
The photo library has photos from 2 iPhone 4s, 2 Leica and 2 Nikon cameras of varying sizes. The problem does not seem to be associated with any particular camera.
User 2 (my wife) is very unhappy!
Any help and insights welcome.
Many thanks
Wally

Answer 1

Where is this library located? If it's in the Shared folder then that's probably why you're having problems. But it's usually with permissions, not broken links/file path to the original photo that the exclamation mark is an indication of. 
Your best and most trouble free solution is to put the library on an external hard drive that's been formatted OS X Extended (journaled) with owner ship set to be ignored:
You can try the following to fix the problem but be warned that the problme may manifest itself again after some use: 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 #1, followed by #3 and then #4 as needed.
Fix #2
Using iPhoto Library Manager  to Rebuild Your iPhoto Library
Download iPhoto Library Manager and launch.
Click on the Add Library button, navigate to your Home/Pictures folder and select your iPhoto Library folder.
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
In the next  window name the new library and select the location you want it to be placed.
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 but not books, calendars or slideshows. The original library will be left untouched for further attempts at fixing the problem or in case the rebuilt library is not satisfactory.
OT