If it's any use, I found this, it's a document about the world heritage site from English Heritage / Bournemouth university
http://csweb.bournemouth.ac.uk/stonehenge/pdf/section2.pdf

It looks like there have been three such plaques found in the stonehenge area.

It mentions the 'plaque pit' on King Barrow Ridge, as tiompan says - "One of the two chalk plaques from the Plaque Pit carries an incised image in the form of an opposing Greek key pattern set within a tram-line frame; the other has a cross-hatched design within a tram-line frame (Harding 1988)."

and Harding's article is this, if you can find it no doubt there is a picture
Harding, P, 1988, The chalk plaque pit, Amesbury. Proceedings of the Prehistoric Society, 54, 320-27

and similarly, there's the original article from when the road was widened and the plaques were found:
Vatcher, F de M, 1969, Two incised chalk plaques near Stonehenge Bottom. Antiquity, 43, 310-1

Also there was a similar plaque found in a pit at Buttermere Down near Amesbury -
"The example from Butterfield Down also has a tram-line frame, the interior being filled with parallel lines"

and that comes from
Rawlings, M, and Fitzpatrick, A P, 1996, Prehistoric sites and a Romano-British settlement at Butterfield Down, Amesbury. Wiltshire Archaeological and Natural History Magazine, 89, 1-43

Carbon dating of other things in the pits dated the plaques to the early third millennium BC, and the document on the link suggests there's a link between the really early Grooved Ware pots they found and the patterns of the chalk plaques - and of course all the patterns of rock art on stones and tombs elsewhere?

So no pictures sorry. depends what sort of libraries you've got nearby and how much time you want to spend to find these elusive plaques I guess.. and where are the originals!