Error in retrieving data most likely. You have hardware (like the graphics card) and then you have software (which makes it possible for the computer to read/use the hardware). A computer is calling up information all the time - lots of information is the usual case - it is not too uncommon for a computer to mis bits of data, or even to not find the location (address) of data on your hard drive. This can lead to physical effects like missing photos/images. One way to insure that this happens less often is to regularly clean your disk (cleaning out unused and unwanted files like the recycle bin, cookies, etc) scan disk (checks for errors and damage to the disk if found the computer will write a subroutine to no longer use that area) and then a defragmentation which shuffles around bits and pieces of programs - most often a single program can be broken up into chunks each chunk located in a separate space on your hard drive defrag pulls together these chunks, and places all the movable files in specific locations which makes it faster and easier for the computer to find data - it also frees up a lot of space on the hard drive. Another way is to insure that you have the latest drivers for your hardware. I personally run diskclean-up, scan-disk, defrag on my computer every 3rd Saturday of the month. On top of that I check for updates for all of my hardware (which means going to the manufactures site for specific things like sound card, graphics card, printers, etc.) My computer is 7 years old, yet still runs like a 'new' computer - fast, efficient and rarely makes errors.