I saw several topics about this warning message related to the simple slider. I turned off all my plugins and rebuilt my page, checking at each step to see if it came back, and it did. I’ve solved the problem but wanted to let you know WHEN it showed up:
When I uploaded an image that was a different size from the others I’d uploaded.
Removing the photo that was a different size and resaving the slideshow solved the problem. It appears that the site won’t crop images, and it won’t allow different sized images using the “No Scaling” selector, either; either of those resulted in the warning message.