1 down, another one almost.
The willows now have "real" leaves, not just some colored planes for leaves! The method described by Ulco was what I did use in an earlier stage but produced weird results; that's why I gave up on that and did not use any images. Stubborn as I am and encouraged by the advice however I did return to the drawing board and started fiddling around with different leaf images and that gave me a clue finally:The images were pasted on to the leaves under a wrong angle... I rotated a leave image 90° (though it wasn't even a willow leaf) and got something that actually resembled a willow leaf. See "Detail Knotwilg".
The daffodils however are a different story. The object has five individual shaders for the petals and one for the trumpet so, following the same method as used with the willow leaves, I did assign an image shader to both the color and opacity image ports of those shaders. Also I gave them slightly different shades of yellow. The result was disappointing.
So I moved into unknown territory: I did attach a PF to the color function port of the petal shaders and used very small values for "feature", "lead-in" and "smallest" scale and unchecked the "Apply displacement" box guessing this was what Ulco meant. It made the petals look like they had a disease. Setting bigger values didn't help either, I could not see a difference except the spots on the petals grew larger.
So I removed the PF and used more stronger color variations on the petals; the result can be seen in the "Knotwilgspoor-LenteREV_RfDetail" image.
I don't give up on the daffodil, will experiment some more, even though the next version of the image will have the daffodils more in the background.