Grab known screen capture ratio

Problem: I have thousands of images that I want to use on my website. But I only need part of each image, and I need that part to be a consistent aspect ratio. So far, I've used any of several screen grab utilities to select the desired portion at the required size, but this is really tedious.

I need the ability to move the selection (all the while maintaining the aspect ratio) around on the image in order to get the desired part. The needed images needn't have resolution greater than screen (meaning screen grabs are one approach). I've been trying several approaches:

1. Grab from the screen: There are many excellent ways to do this; Apple's Screenshot is elegantly simple, and very close to a solution, but will not remember the size of a selection rectangle, nor allow the selection window to be moved once it is placed. Curiously, Screenshot has an option to 'Remember Last Selection'; I don't know what this controls; to my knowledge, it does not control the size of subsequent selection windows.
2. Crop an image: the key thing here again is for whatever app to remember the aspect of the desired crop area.

I've tried so many apps; at first, I tried various screen grabbers. I also tried GraphicConverter, Preview, …it's a long list. Seems simple, and I'd prefer an Apple app, and hoping anyone else has this issue?