I have applied such image textures before without any problem but with the eyes of this character I simply cannot get the image texture to appear in the 3D viewport in either Eevee or Cycles. I feel sure I have done something daft or not done something simple but I have gone over everything several times and searched online yet cannot find what I am doing wrong. I have attached an image showing my setup. All help appreciated.
I am closing this request for any further comments since no more suggestions have been forthcoming and nothing I tried made any difference to the lack of display in the viewport. I reluctantly deleted the eyes and made new ones applying the same image texture, in the same way and they work perfectly well. I guess I will never know what the problem was. Thanks for looking.