  Within quantum-gravity approaches and beyond, different mechanisms for
singularity resolution in black holes exist. Under a set of assumptions that we
spell out in detail, these mechanisms leave their imprint in shadow images of
spherically symmetric black holes. We find that even current EHT accuracy is
sufficient to place nontrivial constraints on the scale of new physics within
one modified spacetime, if the EHT measurement of M87* is combined with an
independent measurement of the black-hole mass. In other spacetimes, increased
accuracy is required that the next-generation EHT may deliver. We show how the
combination of $n=1$ and $n=2$ photon rings is a powerful probe of the
spacetime geometry of regular black holes, even when considering astrophysical
uncertainties in accretion disks. Further, we generate images containing a
localized emission region, inspired by the idea of hotspots in accretion flows.
Finally, we investigate the photon-ring structure of a horizonless object,
which is characterized by either two or no photon spheres. We show how photon
rings annihilate each other, when there is no photon sphere in the spacetime.
