Camera equipment is frequently awful for depreciation, as I know only too well and everything has changed with digital. I think a lot of modern cameras also have built in obsolescence at a mechanical level with no option to repair after a few years when something fails.
I’ve been giving this some thought and I’m not sure this is really a problem of saving the image at 2500px or 3500px. The bigger issue is choosing the right image with enough flexibility, which essentially means a lot of space around the central subject, so you can easily crop at almost any point.
In terms of file size, large desktop images can become huge, even when heavily optimised. This is especially true if they are highly detailed or noisy, let alone including film grain. Obviously a fast server helps, but I invariably create separate versions for mobile and tablet as well. For image compression I’ve always saved for web using Photoshop, but have to admit ImageOptim has improved greatly and it saves a lot of time.
On the website with the image of a model lying on a beach, I eventually lost patience and replaced it with an image of an eye. This was originally cropped vertically, so I ended up having to extend the skin area around the eye in Photoshop considerably. You have to pick your images carefully, but it’s obviously easier to test if you have a large display.