Some pictorial symbols have become, over time, almost universal— usually not because their meaning has been uniformly and consistently interpreted, but because their intended meaning has been learned.
This is a branch of an earlier mentioned issue of choosing images that as clear as possible and can not interpreted as anything else. Using icons can be tricky in itself, but there is not reason to "reinvent the wheel" when choosing an icon to represent something. It might be more visually appealing or a clever, but it could perceive as something else. For example, a icon of a letter on a screen or the "@" symbol has been universally acknowledged as two icons that indicate email. But if you try to create another icon to represent email, then the audience might be looking for the universally acknowledged icon and miss the one that is new to them. Using icons that everyone knows and acknowledges ensures that your website is easily navigable.