Hi, I am Dave, I will help you with this.
To reduce the size of an image file basically means you need to remove pixels from the image, either by reducing the dimensions of the image or reducing the resolution of the image file, both will reduce the clarity of the file.
In Windows, there is no built in app in Windows that can reduce the resolution, only the dimensions of the image file, you could use a 3rd party image app like the free version of IrfanView to reduce the resolution of the image files.