If you have not done so already, you can download TouchCopy from here.
TouchCopy will copy the largest copy of a photo it can find on your iPod or iPhone to your PC. The size of this image depends on the type of iPod you are using.
There is an option in iTunes to copy full resolution photos to your iPod when syncing - if you had set this option TouchCopy will be able to copy the full size photos to your computer.
The image below shows the setting required in iTunes when syncing photos on your iPod:
Please note that this option is not available for the iPod Touch or iPhone devices and so iTunes will always copy scaled down versions of your photos to your device.