I am not getting photos to show up-help!?

When I turned on my computer today, some photos didn't show up and just a box with a red x in its place. I went to Ebay and most of the photos of the items were also just red x's. How do I get the pictures back? I am what you would call "computer illiterate" so any info in detail would be greatly appreciated!!! Thanks so much-I will pick best answer!