The influence of static atomic displacements, due to atomic size effects in alloys with atoms having different covalent or ionic radii, on high angle annular dark field image contrast is studied quantitatively by simulations and experiments. We show that the static displacements can have a large influence on the Z contrast, depending on the alloy composition and on the scanning transmission electron microscopy specimen thickness. This influence has to be taken into account for quantitative chemistry measurement based on Z-contrast imaging.