The picture concepts were low on my son's first test because he draws stick figure dioramas rather than detailed, pretty art. The tester decided the drawings were basic, rudimentary, and poorly constructed. Some of that really is subjective.
I just wanted to address this part of a response. There are no written or drawing tests on the WISC at all. The only parts that involve a paper and pencil are the two PSI (processing speed) tests: coding and symbol search.
Picture concepts is described as,
Picture Concepts measures categorical, abstract reasoning. Students are asked to look at two (or three) rows of pictured objects and indicate (by pointing) the single picture from each row that shares a characteristic in common with the single picture(s) from the other row(s).
Picture Concepts is an untimed core Perceptual Reasoning Subtest.
In regard to the OP, my oldest, whose processing speed was much lower than her other indices has done very well with acceleration and GT placement. I certainly would not see lower speed as a preclusion for acceleration.
Possible accommodations you can ask for, and that we've seen work for our kiddos, include:
-a grade skip
-subject acceleration (will they do this for any subjects other than math where he's already getting this?)
-extra time on tests if needed in order to place him at the appropriate level and have him perform well
