"If you add it to the base/diffuse colour it's still affected by other contributors to the final colour, most notably in this case SSS. You need to use the same image as a mask on SSS strength or, more simply if you are using Iray with the Iray Uber Base shader, apply your tattoo to Diffuse Overlay."

Where is the DiffuseOverlay? There is Diffuse Overlay Color, Diffuse Overlay Roughness, and a few others starting with Diffuse Overlay- none of them are just "Diffuse Overlay". Placing images LIE on any of those changes nothing.