The files are quite big. I got them from https://www.wuala.com/Olson/Photos/Optikz_360_Skies/. I discovered them while searching for HDRI images to enhance my skies in Kerkythea. A nice guy made a post on the Blender Artists site as a gift http://blenderartists.org/forum/showthread.php?224065-New-High-resolution-sky-pack-for-Blender.
I got 9 out of this list including:
brightday
cambridge
english
godrays
goodmorning
Nevada
clearday_1
clearday_2
Sunset
The last 3 are png files. I opened them in Photoshop and saved as jpegs just to keep the import list simple. But I don't think there is a problem with any of the image files at all because all of them imported successfully, just not all at the same time. The problem does not seem to be with any particular image, just which ones get imported last. But once an image gets flagged as invalid, I cannot import it anymore no matter how many images I purge from the model. I'm thinking of splitting the library up into 4 skies each since the size seems to be a problem for some reason I think.