Images not appearing in Google's cache?

[FlashDriveDT]

I just noticed that some of my pages dont show the images in google's cached version. These are fairly recent cached pages, some even today (April 17). Some of them show the images in the cache, some don't. Of those that don't, once I click on the normal view of the page the images load fine. What could this be? The only pattern Im seeing is that the cache times where the pages dont show the images, correspond with the W3-total cache plugin being active. Thats a wordpress plugin that makes caches of the pages, including images, to make everything load faster. I guess this causes a problem with Googles cache? Do I need to worry about this? I'd really like to keep this plugin with its fast loading times.