Windows compresses wallpapers by default. To see the wallpaper in full quality, you should disable desktop image wallpaper compression. Here's how.
Every time you set a new wallpaper, Windows automatically compresses the image. In case you don't know, a compressed imaged will be of lower quality than the original. Now, Windows doesn't compress the image to a noticeable extent. However, depending on the image, the sharpness and detail of the image will be reduced to some extent. By 15% to be exact. If you are eagle-eyed, you can definitely see the difference between the original image and the wallpaper. This is applicable to all images with JPEG extension.
Windows does this from way back when to improve desktop performance. Back when the computers are running with a couple of gigs of memory and single or dual-core processors, compressing the wallpaper might have given a teeny tiny bit of snappiness or performance. However, modern computers are pretty capable and good high-quality wallpaper doesn't affect the system. Not to mention, having a crisp wallpaper with no image compression gives your desktop a neat and punchy look. Especially on high-resolution monitors.
So, if you don't want Windows to automatically compress wallpapers, follow the below steps. It will guide you to disable desktop wallpaper image compression in Windows.
Note: You can follow the below steps in Windows 7 and 8 too.
Disable Wallpaper Compression via Registry Editor
Before moving forward, it is important that you back up the Registry to be safe. If you make any mistakes or if the result is not to your liking, you can easily restore the registry and get back to the previous state with just a few clicks.
1. First, open the start menu, search for "Registry Editor" and click on the result to open the Windows Registry Editor. Once the Registry Editor has been opened, go to the following folder. If you are using Windows 10, copy the path, paste it in the address and press Enter to go to the folder immediately.
2. Here we need to create a new value corresponding to the wallpaper image compression functionality. So, right-click on the Desktop folder and select the "New → Dword Value" option.
3. As soon as you do that, an empty value will be created on the right panel. Name the value as "JPEGImportQuality" and press Enter.
4. After creating the value, double-click on it, select the "Decimal" option, type "100" in the field and click "Ok" to save changes. In case you are wondering, the 100 value tells Windows not to compress the image. i.e, show the image with 100 percent quality.
5. Close the Registry Editor and reboot Windows. After restarting, re-apply the same wallpaper or new wallpaper. If you want to, you can also apply new wallpaper every day in Windows.
As I said, you might not see the difference instantly but you can rest assured that the wallpaper compression is disabled.
Revert the Changes
To enable the image compress again, simply delete the JPEGImportQuality value. Alternatively, change its value data to "85". 85 is the default value. Windows compresses wallpapers to 85% of the original quality.
I hope that helps. If you are stuck or need some help, comment below and I will try to help as much as possible.