At the most fundamental level, the difference is that when you place an image, even from resources, using the Image stack it creates a unique version of that image. If you copy that same Image stack to another page or place it in a partial, even without making any changes to the settings, etc., it will create a unique image everywhere. A browser would need to download each image, even if it’s identical.
Anywhere you use a Site Image with the same source image, it always points back to that same file. Once a browser downloads a Site Image, it does not need to download an image for each instance. It’s just the same image file displayed again. If you use an image in multiple locations, Site Image will load images much faster after the first one is loaded.
RW is not an image editor. Yes, it has some image editing and cropping features built in, but you are much better off using a dedication image application for editing/cropping, With as many images as you have, I would not want to be editing my images in RW.
Look into batch editing and resizing application. There are many. One of which is Squash, made by RealMac.