Alrighty, in the absence of more informed answers:
I think you’re right that Lovecraft only described Cthulhu in The Call of Cthulhu. The description there reads:
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 hind and fore feet, and long, narrow wings behind.
Your image seems to match that pretty well, although I wonder if the arm fins and little claw bits on the wings were drawn from other mythos authors. (I’m not familiar with non-Lovecraft mythos works, so I’m not sure.)
It’s worth noting that H. P. Lovecraft did his own sketch of the Cthulhu idol described in The Call of.... It ties up reasonably well with your image, but perhaps also demonstrates why Lovecraft didn’t illustrate his own publications:
For fun comparisons with other fictional works, check out this page, a great example of why Wikipedia exists.