How to Repeatedly capture fixed screen region to sequence of images

I need to repeatedly capture a fixed region from each of a sequence of web pages to a series of images that I can then compile to a PDF. Ideally I could just navigate from page to page and press a hot key to capture the area and save the image to an automatically incremented filename each time.

What I want to stop doing is manually defining the region each time (it's irritating and when I drag to create the region it's even more irritating to try to get the exact pixel dimensions correct), which seems to be the only option with Skitch and others I have tried.

What software will allow me to do this most efficiently?