We used the Precision Integrated-Optics Near-infrared Imaging ExpeRiment (PIONIER) at the Very Large Telescope Interferometer (VLTI) to image the stellar surface of the S-type Asymptotic Giant Branch (AGB) star π1 Gruis. The angular resolution of two milliarcseconds allowed us to observe the surface of this giant star in unprecedented detail. At the observed wavelength the stellar disc appears circular and dust-free. Moreover, the disc is characterised by a few bubbles of a convective nature. We determine the contrast, and the characteristic horizontal length-scale of the convective granules. The latter is determined, for the first time, directly from the image, without involving the usual geometric modelling that has been used in the literature. The measurements fall along empirical scaling relations between stellar parameters and convective sizes, which are determined on the basis of three-dimensional stellar convection models. Our results open up a new era for the characterisation of stellar convection in stars other than the Sun.