Site Speed and Image Sizing

Site URL: https://www.asiawest.net/

Hi there! I have a question about site speed. I'm using Google's Pagespeed Insights feature and noticing that many of my pages are not optimized to load quickly. Aside from the suggestion to "Eliminate Render-Blocking Resources", which I've read is not something I can change, the biggest things slowing down my site are my images not being in "next-gen format" and not being properly sized. How do I know what size is optimal to save my images at without compromising image quality? Is this something I can do in Photoshop when I go in to save my images in a new format (JPEG 2000, as Google suggests)? I am kicking myself for not doing this earlier and not having images properly sized before uploading, but I'd like to resolve this as painlessly as possible now. Any help is greatly appreciated!