I have a dozen or so image files on my iMac's desktop. Most of the time, after booting up my computer, it displays a thumbnail of each image (as it should). But sometimes, the thumbnails won't show up (see screenshot below).
It appears to happen randomly. I estimate that it happens about 20% of the time.
Why is this, and how can I prevent it from happening? I am running OS X 10.9.4. Thanks.