Lovecraft was not an imagistic writer. The stereotype is one of vague adjectives: abnormal, unutterable, monstrous. And true, we're told of Great Cthulhu:
The Thing cannot be described - there is no language for such abysms of shrieking and immemorial lunacy, such eldritch contradictions of all matter, force, and cosmic order. A mountain walked or stumbled.
But consider what else is said of Cthulhu, when the Great Old One first rises from R'lyeh: its gelatinous green immensity, the green, sticky spawn of the stars, the flabby claws, the mountainous monstrosity, the pursuing jelly, the awful squid-head with writhing feelers. And, with the most pungent adjective in all of Lovecraft, great Cthulhu slides greasily into the water.
Earlier in "The Call of Cthulhu" we have been shown both a bas-relief and a small carving of Cthulhu, the latter of which furnishes a particularly concrete example:
It represented a monster of vaguely anthropoid outline, but with an octopus-like head whose face was a mass of feelers, a scaly, rubbery-looking body, prodigious claws on his hind and fore feet, and long, narrow wings behind.
In this way Cthulhu's shape is delineated long before we see it. Later, when we view the Thing itself, Lovecraft feels no need to describe Cthulhu further, except inasmuch as the creature differs from its image: Therefore he tells us of Cthulhu's mountainous size and - confirming his "rubbery-looking body" - his disgusting softness and pliability. When Cthulhu finall appears in the story, Lovecraft is able to elaborate on the meaning of his physical presence, and how it destroys the lives and sanity of those who behold it...
For the rest of Pat's introduction, pick up a copy for youself!