Keep quality when importing image to appearance

To create a realistic timber (white oak) appearance I am changing the image file of the appearance. I start with the in-built 'polished oak' appearance and change the image file to a high-quality photograph of oak. This is working fine apart from the quality of the 'image' on the part is not as good as the original photo. This means quality and definition is lost on the oak grain. Does anyone have any ideas?

Thanks for any help.

SolidworksPhotoview 360