Since your iPad Photos library can be accessed from a computer, it is natural to assume that you can reverse the process and import pictures from your computer to your iPad, going through Windows Explorer in Windows 7 / Vista / XP, Finder in Mac OS X, or Nautilus (for example) on Linux, Ubuntu or other distribution. You also know perhaps, that iTunes allows you to synchronize pictures from your computer, and add them to your iPad's photos library - but for this tutorial, we focus on using your iPad as an external storage device.
When you try to paste the image files you copied from your computer, Windows Explorer (for example) will have the Paste command disabled (grayed out), preventing you from transferring these image files through pasting. If you try to right-click to move, you will get the "action forbidden" symbol, as shown on the screenshot below: (icon next to the mouse pointer / cursor)
We have not yet tested the drag-and-drop technique from an Apple computer running Mac OS X, or a PC running Linux. The result is likely the same, although Mac OS X may have a tighter integration between iPad, iTunes, and Finder - and guess what you are trying to do, etc. Remember that everything we are trying to do can be done, in part or in whole, while going through iTunes itself.
A later tutorial will explain how to sync' image libraries between your computer and your iPad by going through iTunes.