How does a name make a beam of light or an image. Not everyone needs to think of what something looks like. You can worship something based purely on name, actions, and teachings. And as far as islam goes, it's like Judaism. Jews hold the name of God so highly, that they try to avoid writing it down, in case someone desicrated god's written name or in case they misspell it, same for saying God's name. So transfer that to God's image for Islam. If you misrepresent God, even if it's only by how he looks, it is an insult directly to him. You don't want to misrepresent the most holy being in existance who created everything, do you?