I've just run some analysis on the 4487 scene images that get run through the conversion process to calculate their SSIM as a measure of image quality. I'd read somewhere that 0.97 and above was a good quality image.
Uncompressed build
1106 x Lossless, 280 x SSIM > 0.99, 3113 x SSIM 0.98->0.99, 386 x SSIM 0.97->0.98
The "worst" image in the uncompressed build is apparently
Dream_2: Original 1.2Mb PNG, webp is 139Kb, 11.4% of the original size (SSIM 0.972)
View attachment 2806106
Here's a closeup on the face, original on left, compressed on right
View attachment 2806077
You can see some tiny differences in the hair if you really look for them, but the changes are almost imperceptible; and this is the worst quality image in the uncompressed build. I don't think it's worth the extra download to go lossless.
Compressed build
8 x Lossless, 1309 x SSIM 0.97-0.99, 2919 x SSIM 0.94-0.97, 609 x SSIM 0.91-0.94, 42 x SSIM > 0.88
The "worst" image in the compressed build is apparently
JimSlater5_11: Original is a 1.4Mb PNG, the compressed webp is 56Kb, 4% of the original size (SSIM 0.886).
View attachment 2806108
Here's a closeup on the faces.
View attachment 2806086
Here, you can see differences. The faces have been smoothed, the detailing on the shirt and the background is now blurred and Stacy's hair has lost definition. Still, considering it's the worst image, it's not too bad.
I've uploaded all of the images here for reference (both the original lossless PNG and the uncompressed/compressed WEBPs for reference):
Workupload
tl;dr: The compressed build image quality is high despite being 22% of the size of the original. The compressed version is not too bad considering it's 5% of the size of the original.