Icon: which bears a direct visual relationship to what it denotes.

Index: which forms a stable collocation with the entity that it denotes.

Symbol: which has an arbitrary form and denotes its referent by pure convention (see sign).

In an advertisement, any image of the product is an icon, representing the product more or less accurately.

An index in an advertisement is generally quite culturally-bound, representing something by association. A typical example might be a head of a wealthy-looking woman in a diamond advertisement (an index for a wealthy lifestyle) or some friends laughing in any kind of advertisement for a social product (e.g. fun cameras, drinks, cigarettes, certain kinds of clothes). We are unlikely to see three laughing teenagers in a diamond advertisement, or the head of a glamorous woman in a disposable-camera advertisement.

These indexical components of advertisements have connotations, which typically connect the advertisement to a larger cultural context.

Symbols need little illustration; everyone can easily think of company symbols. (The McDonald's "Golden Arches" is of course originally indexical from the name.) In terms of cultural significance, a company is well-served if its symbol becomes an index---a signifier which goes beyond what it directly signifies to some larger association. The symbol, too, may come to be indexical over time. This represents a very strong cultural establishment of the symbol, and may be a very powerful marketing tool.

To summarize, within an advertisement, the Image component may have some part that is iconic to the product, and there may be some symbols such as a company logo, and so on. The overall Image may have some cultural associations, which are indexical to some larger cultural context.