In this work, Scanning Tunneling Microscopy (STM) images were simulated for the organic molecule 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA) to study the effect of the number of tunneling states and the integrated Local Density of States (LDOS) isosurface value. Local Density Approximation of Density Functional Theory (DFT-LDA) calculations were performed to achieve the simulated images under the Tersoff and Hamann approximation. The number of tunneling states has a strong effect on the image appearance of the patterns. Intermediate isosurface values for the integrated LDOS produce good resolution and matching in relation to the experimental STM image. Both parameters seem to be of significant importance for STM image simulation.