You are not just seeing the pictures you've taken, but also thumbnails that Android creates to allow quick previewing of the picture in gallery and elsewhere.
The pictures you are seeing from your Internet browsing are also saved by Android the same as a normal computer would save temporary Internet files and cookies. You can clear these yourself by going into your Internet settings, and clearing Cookies, Cache, etc...
All you have to do is go to the actual folder where your pictures are saved, rather then allowing your computer to show you every type of image file that's saved on the SD card.