Zoom in on the HST WFC3/F606W image to highlight the ASASSN-15lh host galaxy morphology (left panel, logarithmic scaling) and the transient (right panel, linear scaling). Both panels show the same image, sky area, and contours, but have different color bars. The image size is approximately 4.′′2 by 4.′′2, which is 16 kpc by 16 kpc at the redshift of ASASSN-15lh. North is up and east is to the left, and a physical scale at z = 0.232 is indicated in the bottom right corner of the image, respectively. The position of the transient is indicated with a white cross in the left panel.