Why are my pictures showing up as blank in 'faces'? ...There's only a dotted line around the edge of the space where the picture usually shows up?

If you click the area between the dotted line, can you see the photo?

Then probably the thumbnails in your iPhoto library are corrupted.

Assuming your iPhoto version is 9.5.1 - if not, post back.

Try to rebuild the thumbnails:

• Hold down the option and command keys (⌥⌘) at the same time and double-click the iPhoto icon.
• From the window that will pop up use the "rebuild thumbnails" option.

If you do not have a current backup of your iPhoto library, it would be safer to make a backup, before you try this.