Have you ever been browsing a thread on Ponychan and went, "wow! I really like all the images in this thread! I wish I had a script that I could point the board moniker and thread ID to and have it download all of the images in the thread for me, with built-in duplicate image protection and automatic file extension checking!"
Well, for one, that's a very specific thing you just thought to yourself. Second, such a thing now exists.
It comes in Windows, OS X, and Linux flavors.