When viewing files in thumbnail mode in Windows Explorer, there is a limit. In Windows XP, Explorer can only show thumbnails for up to 4,096 (4K) files. After that, files begin showing either a completely white image or the icon of the program associated with that file-type.
Which files are given a thumbnail or not is determined by the order in which they are displayed. For example, if a folder containing 5,096 files is put in thumbnail mode and the first 2,048 files are viewed, then End is pressed and the list is scrolled up, the first and last 2,048 files are given a thumbnail and the middle 1,000 files are left blank. (Of course if the list is scrolled around with the scroll-bar, then seemingly “random” blocks of files here and there may be left blank.)
(There may or may not be a limit in Windows 7 as well. A test will be done and results will be added.)