That way their access is limited to when you sign on for them. If you subscribe to one of the major on line services listed above, you can send Email to the customer service department asking to lock out access to areas with adult content. Local computer bulletin boards are often sources of adult material, especially smaller ones run by teenagers. Larger bulletin boards tend to have better security, and have more at risk if they receive complaints. Another way to see what your child is up to is to know what's on your hard disk and floppies.
Graphic images are usually end in .GIF or .BMP, but there are a number of others.
Use a graphics viewer to view the photos. Hopefully there won't be any surprises. At least four manufacturers are introducing software that parents can use to block access to sexually explicit pictures or harassing conversations on the Internet and other on-line services. Parents can then make the determination whether or not files need to be block. The second step allows parents to block files.