Voltage-dependent scanning tunneling microscope (STM) images have been calculated using density functional theory (DFT) with the general gradient approximation (GGA) for Five chemisorbed forms of acetylene on the Si(001) surface. The effect of electric field between the tip and sample is addressed by a high-voltage extension of the Tersoff-Hamann approach. The calculated images are used to aid in assigning the species observed in recent STM measurements on the Si(001)/acetylene system. An image which had been previously attributed to a tetracoordinated acetylene molecule is reassigned to a site with two acetylene molecules, bridging two adjacent SiSi surface dimers.