It really depends on what your long range plans for them are. Here's my thoughts:
1. Are these pics that you will access off and on? If not, and they are pics that you just don't want to lose, I'd put them on CD or DVD. This would be my choice for a more permanent storage option. This will require more media (cd/dvds) than the other options.
2. If you are going to access them often and only on your computer then an external hard drive is a good option. Keep in mind that if you get a new computer in a few years, and it has new technology, the hard drive may not work on the new one and you would then need to transfer them. I had a friend that ran into that problem.
3. On the other hand, if you have more than one computer that you go back and forth with, then a flash drive is a good option, but I would not use the flash drive as a permanent option.
In the end, any of the options will not only free up space on your computer, but preserve them in case something happens to it. When my laptop died, I thought I'd lost years worth of pics. I still haven't transferred them off the laptop or the desktop. I need to do that.
Just my thoughts.