This is not so much an issue about the dimensions; it’s the format you’re using.
PNG is great because it preserves transparency, but even if you scale these images images to 400x381 as suggested above, most will still be sized at well over 200 KB, meaning the total assets could reach over 2 MB. Of course, at the current size, some of them are 3 to 4 MB each! 
Here’s the thing: your website’s background is white—this means you don’t need that transparency! The same assets using high quality JPEGs at 400x381 are less that 50 KB each, saving as much as 80% compared to PNG and almost 99% compared the original, larger PNGs! 