I'm not exactly breaking new ground when I say art is powerful.
Sight is the king of the senses in humans (er, “us humans” I mean), usually riding roughshod over weaklings like touch and hearing — we don't have both eyes on the front of our head for nothing. Unlike text, which requires an intermediate “translation” before it gets turned into a picture in our head, pictures go straight to the source. Which is why we say things like a word is only worth 1/1000th of a picture. This whole post is only worth about half a picture.
Now I'm not an artist, I'm a writer. Well actually I'm a GM, which isn't even the same thing because a GM uses the written word as a launching point for what is basically performance improv, the final product being the game not the page… but I digress.
Since I'm not an artist, when I make material for publication I have to hire artists to turn my words into pictures. If the artists I hire are any good (and I try to ensure they are) this can be a very dangerous process — dangerous because if an artist produces something that looks great but doesn't match what I had in mind, I have to resist that change and stay true to my original concept.
Once something is drawn (and drawn well), that image becomes very compelling. It can be very tempting to let an image I see replace the idea I have in my mind. Holding true to the original concept, resisting the power of art, can be very difficult.
Usually a character has existed in my mind long, long before I see an artist's interpretation. A character could be in play for years before it is put into a draft for publication and an art spec is sent out — Dreadnaut was first used in-game three and half years before the description was sent to an artist.
That's a blessing and a curse. On the plus side, you have a strong mental image of the character, so you have a little more strength to resist a “wrong” image.
On the downside, you have a strong mental image of the character, which can lead to excessive perfectionism (“but it doesn't look like I imagined! Wah!”).
Sometimes there are details that are critical to me because they imply something that will actually make a difference in play. An early sketch of Scorpio showed barbs/harpoon tips at the end of her whips. It looked good, but to me it was critical they were removed. Why? Because even if the text says nothing about it, players and GMs looking at that picture will think “ah, we can ruin her whips if we cut off the ends” which went entirely against the concept of her powers: she is supposed to be able to reel out as much cable as she wants, and even leave behind severed sections to tie people up.
These unintentional implications can easily overwhelm the text. Draw a sad person, and it doesn't matter if the text says he's happy. The memorable impression will be that he's sad, and that's how he'll be played and reacted to in the game.