Appearance bitmaps lose resolution when enlarged in drawing

[r.moss]

I have a stone wall appearance, generated from a photo.  The model looks like this:

This is  a small component in a large landscape.  When I make a drawing of the landscape at 1:1000 and enlarge the wall section (1:50), it only appears in low resolution:

It is still in low resolution if I then export the drawing as a pdf.  What can I do about it?

Thanks

Roger