By default, Microsoft Word compresses images to reduce file size. Once images are compressed, the original quality cannot be recovered. To ensure you use the highest-quality images, the image compression should be turned off before saving the document.
You can disable image compression in Word's settings:
PC: File > Options > Image Size and Quality > choose Do not compress images in file and set default to High fidelity
Mac: File > Preferences>Edit > Image Size and Quality > choose Do not compress images in file and set default to High fidelity