Lo-res images

The Genesys entry page https://www.genesys-pgr.org/welcome or https://sandbox.genesys-pgr.org/welcome loads

https://sandbox.genesys-pgr.org/html/1/images/welcome-bg.png

content-encoding:gzip
content-length:780344

The image is almost 1M in size and that takes time even on fast connections.

Can we have a low-resolution image (lo-res) image specified and loaded first and then after page loads, replace it with this high-resolution image?

What approach do you suggest?