Small documents & pictures often move when you close the scanner lid, especially if you close it quickly. Try closing it gradually and slowly.
The centre of the scanner bed is actually the "sweet spot" for the sharpest possible scan, so it's a good place to put small items for scanning. I once had to scan a lot of pictures measuring 6 x 4 inches so I made a template out of cardboard the exact same size as the scanner glass.
Then I cut a 6 x 4 inch rectangular hole in the dead centre of the cardboard, layed the cardboard template on the glass, and placed each photo in turn into the rectangular hole. Of course, both the cardboard template and the photo must be as flat as possible for this to work properly. Close the lid slowly and the photo should stay in the centre, held there by the cardboard template. You can even stick the template down with sellotape in each corner for greater accuracy.
Using that template allowed me to scan all those pictures accurately in half the time it would have taken without the template.