Surely these two sentences either say or imply very different things about the meaning of a picture:
On [the proper] account what a painting means rests upon the experience induced in an adequately sensitive, adequately informed, spectator by looking at the surface of the painting as the intentions of the artist led him to mark it. The marked surface must be the conduit along which the mental state of the artist makes itself felt within the mind of the spectator if the result is to be that the spectator grasps the meaning of the picture.
Surely "the surface of the painting as the intentions of the artist led him to mark it" is misleading here. Obviously we aren't going to first toss some cranberry juice on a Klee and then wonder about its meaning, and the canvas of the painting is the one hanging before us, marked as it's marked. (Maybe what the phrase means is that that the informed, sensitive spectator will look at the signature, since that is the likeliest part of the surface of the painting to have been marked by the artist. But that seems implausible.)—the quotation is from Painting as an Art, btw.
I also found the claim that the original Ur-painting, simply setting down marks, and "guided solely by the thought of the mark", "is an impossibility", odd. Don't people doodle? And, in doodling, hit upon features of what they're doing in a way that seems to match what Wollheim means by thematization, except for the end-subordination (another feature that seems odd)?