I made a thread similar to this about 2 years ago and I am wondering if this is still the case? I once installed Windows 8 and reverted back to Windows 7 just because of the wallpaper issue.
I like using the following sample image. W8 always kept compressing the image and making it look terrible. The before and after picture should be obvious, especially if you look at the red lines.
before (original)
after setting as wallpaper
So does this still happen in W10 so far? I'd really like to know without investing too much time. Thanks~