Image Size & Resolution for Website?

Wondering what would be considered as acceptable for Website uploads?
I've been thinking 3000px on the longer edge as the max at 72ppi for galleries
and 640 for images within posts?
Obviously the interest would be quick loading time & highest quality permissible.

On another line of thought, if i exported an image from LR, say at original dimensions but at 72ppi.
Then re-size the image to 3000/1000/640 (as per requirement) & at this stage too the software wants to keep it at 72ppi.
so does a take a 72ppi image and again re-sample it down further or would it leave it as it since it's already at 72ppi?
If it does re-sample, am guessing the quality would deteriorate further?

Depend in what kind of website. For just viewing 1000 to maybe 1500px is probably big enough.

If you keep up on this kind of stuff, just about every source on the planet agrees that mobile is the next wave. There are now more people surfing the web on their devices than sitting at home in front of a PC. Having a mobile-friendly website design is key...especially for an image intensive business like this.

People ARE going to be looking at your stuff on their phones and tablets, and unless you learn how to optimize for mobile, your pictures will look like crap. Forget about pixels. Erase it from your memory and replace it with %. Pixels are a hard number, so if someone is trying to view your site on a mobile device, your photos will be enormous and throw your entire site design out of whack. Percentages re-size your image according to the screen size they are viewed on.

You'll still want to upload images in a large size, but just tweak your CSS to be % based when showing images instead of pixels for width.