Today we're talking about metaphors. Just like similes, but without the prepositions.
There was an episode of Star Trek: The Next Generation entitled "Darmok" in which the crew encountered a race whose language completed confused them. The translators kept spitting out phrases that were interpretable, but not understandable. Things like, "Darmok and Jalad at Tanagra." The Enterprise crew eventually realized that these aliens' whole concept of speech revolved around metaphors. They used the example: "Romeo and Juliet at the balcony." That phrase will almost immediately conjure a particular scene and mood and set of emotions, but ONLY if you happen to know the story of Romeo and Juliet!
How about an example from music? You ever see somebody's update on Facebook or Twitter that just includes a few lyrics from a song? It speaks to that individual's tastes in music, of course, but the song itself also carries with it a series of meanings and emotions. The song lyrics, even removed from the full context of the music, carry a connotation with them and can serve as an abbreviated shorthand for the emotional state of the person doing the quoting. They might not be literally feeling or expressing the exact words they cite, but the overall emotive quality of the song (often, a short 2-3 minute rock song focusing on a single theme) suggests whatever complex set of emotions the individual might have difficulty putting into words.
The same holds true for visual metaphors as well. Take, for example, this image...
Two girls, one with a violent nosebleed.
Now, if you're familiar with manga and anime, you probably have a pretty good idea of what this scene is about even if you don't know anything about the origins of this particular image. If you're not quite so familiar with them, you likely don't realize that the nosebleed is not literal, but metaphoric. It symbolizes the girl's emotional state. I don't know for sure, but I'd be willing to bet that in the story, no one comments on the nosebleed itself and, in subsequent scenes, there's no visible remnants of the spilled blood on either the girl or the surrounding furniture.
(I won't go into the precise meaning of the symbolism here, but I'll warn people who might try to look it up: do NOT run a search on "manga nosebleed" while you're at work!)
All of the "classic" visual devices in comics follow the same pattern. Motion lines, dollar signs in someone's eyes, thought balloons, a figure breaking out of the confines of a panel... these are all visual metaphors that are only understandable if someone is cognizant of what the metaphor is meant to convey. We don't literally see lines tapering away from someone as they run quickly, but we know the use of such lines implies motion.
My point here is that, as a creator, you need to keep your audience in mind when writing and drawing your work. The nosebleed reference wouldn't work very well in a Superman comic because that audience isn't "trained" in that particular metaphor. That's not to say that you CAN'T train your audience to understand the metaphors you use, just that you need to make sure that when you use a metaphor that might be unusual for them, they might some additional clues to help in understanding your piece.