When you set an image as desktop background, the image is compressed at 85% and saved to a file named TranscodedWallpaper in your Themes directory. In Windows 10, you can override the default wallpaper quality for JPG files using a registry edit.
Set Desktop Wallpaper Quality to 100%
Start Regedit.exe and go to the following key.
Create a DWORD value named JPEGImportQuality
Double-click JPEGImportQuality and set its value data to 100 (decimal)
Restart the explorer shell, or logoff and login back for the change to take effect.
You’ll need to right-click on the image and set it as wallpaper again, so that TranscodedWallpaper in the following folder, is overwritten.
You can note the increase in TranscodedWallpaper file size (and the image quality) after setting JPEGImportQuality to 100. JPEGImportQuality value data can be anywhere in the 60 – 100 range (base: decimal).
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Ramesh Srinivasan founded Winhelponline.com back in 2005. He is passionate about Microsoft technologies and has a vast experience in the ITeS industry — delivering support for Microsoft's consumer products. He has been a Microsoft MVP [2003 to 2012] who contributes to various Windows support forums.