Q: The mystery of vanishing storage - 8 GB

"I purchased a 8 gb ipod touch recently. The storage shows only 6.4 GB available."

That is correct.

"How to use the full quota of 8 GB?"

The OS takes up some space as with any device/computer.

The way memory is reported, just like with your computer, accounts for the rest.

http://en.wikipedia.org/wiki/Gigabyte

How OS X and iOS report storage capacity

You should be importing all pics taken with the ipod to your computer regularly.  You can then organize them and sync whatever you like to the ipod.