The fundamental difference between an image and a symbol is that an image actually resembles what it represents. A symbol, however, neither resembles nor is causally connected with what it represents.
Images are non-arbitrary shapes, the shape of an image is a replica of the actual object.
Alternativley symbols are arbitrary shapes to which we give meaning. Such symbols are combined to form conventions, which are the accepted usage and interpretation of these symbols. However the way symbols are interpreted is heavily dependent upon context, environment, culture etc. No symbol has a fixed or inherent meaning. To English people the symbol CHAT means to talk, whereas the French take it to mean what we call a cat.