What many of our teachers are doing is taking into account the students' learning styles, and, in the case of special education students, the modifications that may be needed for that child and grading according to that - so an excellent picture or diagram that shows how a student solved a problem is acceptable. However, we then take note and try to move students along in the ways in which they are having more difficulty.
One way that has worked for us it to "publish" really good examples of what we're looking for, eg. have two or three children whose explanations and/or diagrams are very clear copy their work onto overheads, show the overheads and have the class evaluate the work, asking "Do you think this is a good explanation? Why?" or "Do you think this drawing shows how Susan solved the problem? Was it clear to you? Why?"