PLEASE do not just move the pictures to the external drive. As one PP already stated, it's not IF a hard drive will crash, it's WHEN. Look at your computer and see if the pictures are truly taking up too much space. Go to My Computer, right click your C drive and go to properties. See how much space is available. If the computer is fairly new, I'm willing to bet you have plenty of free disk space available and you can use the external as a backup drive. If you do not have enough space, consider adding another large internal drive to your computer, storing your pictures on it, then backing that drive up to the external.
My boys are 2 and 5. I got my first digital camera when my oldest was 3 months old. Their entire lives are on my computer. I have logged over 10,000 pictures on my current camera alone. One of my friends once told me they love looking at my vacation pictures because they feel like they were right there with us.

If I lost those pictures I would be devastated! I store my pictures on a 1TB internal drive and back them up to an external. I always back up before leaving for vacation and I either take the external with me or leave it with my Mom - for fear of someone breaking in while we are away and stealing my computer equipment.
I have been in the IT field for 12 years and have seen many people lose all their family pictures due to a crashed hard dive with no backup. It's well worth the $100 to safeguard these memories.