I wonder if you have been (inadvertantly) causing screen capture. There is a feature whereby, if ticked in the appropriate settings, that you can take a screen capture by swiping the side of your hand across the screen, either to the left or the right.
So, maybe this has been happening without you realising, hence then having so many to delete?
Yep, same here. Brilliant mobile, though, don't you agree?
Perhaps we should start an S3 help and tips page ;)
BTW, after posting, I then found out you can take screen shot by pressing the power button at the same time as pressing the 'home' button which is more likely to be done if you are holding the phone and not realising you are putting pressure on the power button at the same time as pressing home button.
Some very interesting and useful videos on Youtube although not always easily followed. Search Galaxy S3, of course