Issue in Windows 10 with lack of icon transparency for System icons

This was a MysticThumbs issue, with old versions that did not support Windows 10 PNG. Newest versions as of version 3.4.0 do support Windows 10. The issue is caused when a PNG from the "Metro" side of things went through MysticThumbs and PNG was set to the old default transparency mode 'Light Checkerboard' they would have the checkerboard background added. These PNG's are now recognised by the newer versions and always made with transparency.

Another option when using MysticThumbs is to either set PNG to transparent in the MysticThumbs control panel, or to disable PNG support in the control panel altogether, depending on your needs.

After fixing things, use the 'Clear Thumbnail Cache' option in MysticThumbs control panel or tray icon and optionally restart Explorer and or Windows to force a flush.

As noted in the comments it appears Windows may have a hidden cache for HiDPI modes that does not get cleared implicitly with a cache clear. Changing your DPI settings to 100% and then back again should fix this.