Molecular monolayer structure investigations have been performed by Scanning Tunneling Microscopy (STM) on self-assembled systems made of dotriacontane, stearic acid, tetracosanoic acid and octadecylmercaptane adsorbed on graphite. High-resolution voltage-bias dependent STM images have been obtained and conditions for optimal image contrast are discussed. A correction procedure to minimize scanning-induced geometrical distortion has been successfully applied on dotriacontane and octadecylmercaptane images.