Technically, via story description, you cannot perceive all of Yog at once. Carter sees no image when he speaks to Yog, because its presence is everywhere. Still, when it "speaks" to Carter, its words are said to translate themselves as "monstrous lights" and "waves". That is the only really accurate way one can depict such an entity, and while I did like the old art, it did not represent Yog-Sothoth.
"Then, in the midst of these devastating reflections, Carter's beyond-the-gate fragment was hurled from what had seemed the nadir of horror to black, clutching pits of a horror still more profound. This time it was largely external—a force or personality which at once confronted and surrounded and pervaded him, and which in addition to its local presence, seemed also to be a part of himself, and likewise to be coexistent with all time and coterminous with all space. There was no visual image, yet the sense of entity and the awful concept of combined localism, identity, and infinity lent a paralysing terror beyond anything which any Carter-fragment had hitherto deemed capable of existing."
^Yog gives off no image.
"And now the BEING was addressing the Carter-facet in prodigious waves that smote and burned and thundered—a concentration of energy that blasted its recipient with well-nigh unendurable violence, and that followed, with certain definite variations, the singular unearthly rhythm which had marked the chanting and swaying of the Ancient Ones, and the flickering of the monstrous lights, in that baffling region beyond the First Gate. It was as though suns and worlds and universes had converged upon one point whose very position in space they had conspired to annihilate with an impact of resistless fury."
^What it is like to perceive Yog's "speaking".