80% is print quality. Web can get away with 30-40% a lot of the time, that helps your Google ranking too, keeping it fast. 400kb images will negatively affect your Google ranking.
If you want professional help you can contact me - Photographers Tech Support. I do SEO, performance optimisation, migrations of all kinds, and pretty much anything else webby. Please don't PM me if you want professional help, contact via my website is far easier for me. A few thoughts below.
Your website gets the lowest score I've ever seen on the Google website optimisation test, desktop section - 0/100 is truly awful, I didn't know it was even possible. Mobile score of 50/100 is fairly poor too, 50% of searches these days are on phones and your website could be marked "Slow Website" in Google results. Background information on SEO and performance optimisation. I tend to optimise front page images individually.
Your website is also the slowest I've seen on this performance test as well - 65 seconds, compared with 3 seconds for my website. Your about page is also poor.
ProPhoto / Wordpress should be resizing your images for you, though you should upload in the maximum resolution you want to share. I personally use 800px, but 1000px is fine too. There's possibly a configuration problem, possibly it's just the way you're using it.
You also have issues around browser caching, which makes looking at other pages faster.
ProPhoto is the worst Wordpress theme I've seen for performance, and yours is the worst instance of it I've seen. There are things you can do to make it better though.