In my last post, I addressed the basic idea for a "visual language" as being a sequence of meaningful images guided by a system of constraints (i.e., a grammar). In the comments, I was asked a very good question:
Why is a sequence necessary for the graphic form to be considered "language"?
There are two main reasons for this, both which relate to the analogy with verbal and sign languages. As I said in that post, my notion of "language" in "visual language" is not metaphorical, but rather based on conceptions from the linguistic sciences.
The first reasons is structural. Languages are made up of three primary components:
1. The conceptual structure of meaning in the mind
2. A sensory modality they can be expressed in (i.e., sound, body motions, graphics)
3. A "grammar" that guides and constrains the sequential expressions of meaning
In the verbal form, the main grammar is syntactic structure, which allows us to sequentially order words and other expressions into coherent sentences. However, technically all of these components (meaning, modality, grammar) are built of rule-systems that constrain them. The phonological system that guides our production of sounds also is constrained by rules. This is why English cannot have words that start with the sound combination "tf" or why the "c" in elastic goes from a "k" sound to an "s" sound in elasticity. These are rules guiding the modality itself.
The analogy for the graphic form holds these same functions. The primary "grammar" guiding images is a "narrative grammar" which guides the presentation of meaning in coherent sequences. Both syntax and narrative function in the same general way: to present meaning in a coherent sequence. They also share methods of doing this, such as chunking units into groupings, making connections between distant units, and embedding groupings inside each other.
However, single images also have a constraining system which is analogous to phonology. You could call it "photology" or "graphology" perhaps. This system similarly constrains the modality itself. This is why certain junctions of lines are awkward, like when you want to show occlusion (one thing in front of another), but instead of using a "T" shaped junction of lines, you use a "Y" or "+" shaped junction.
So, structurally, single images are guided by a rule system, but that system is closer to that of phonology than syntax.
The second reason for sequence being important comes from analogies with development.
By and large, when people are not exposed to a language within the right time period of life, they won't learn language. They seem to be able to still acquire a limited set of vocabulary (i.e., words) but the most problematic component is the syntax.
Even when people are able to learn a spoken language, but never learn sign language, they can still all gesture. The manual modality doesn't disappear as a way to create meaning—it just functions using single expressions without a grammatical sequence.
This same trend is true of drawing and sequential images. Most people cannot draw a coherent narrative sequence. However, they can all use the drawing system ("photology") to create meaningful single images (albeit rudimentary ones if they haven't fluently developed the vocabulary of the drawing system either).
So, the analogy then holds that single images are to visual language what gestures are to sign languages. One type uses a modality for single novel expressions (single images/gestures) while the other uses complete grammars in sequences of expressions (sign language/visual language). The evidence comes because even a rudimentary form of the simpler expressions (single images/gestures) is maintained even if the full grammar is not developed.
Incidentally, quite a lot of my discussion about the structure of single images and the development of the drawing system is available in my recently published paper, "Explaining 'I can't draw'" available as a pdf here.