The present work explores the role of the dilaton charge r₂ and the plasma environment
in explaining the observed images of M87* and Sgr A*. Dilaton charges are associated
with Kerr-Sen black holes, the stationary, axi-symmetric black hole solution in the Einstein-
Maxwell-dilaton-axion (EMDA) gravity which arise in the low energy effective action of
superstring theories. We investigate the impact of the background spacetime (here dilaton
charge and spin) and the plasma environment in modifying the shape and size of the black
hole shadow. The theoretically derived shadow is compared with the observed images of
M87* and Sgr A* which enables us to constrain the background spacetime in presence of
the plasma environment. Our analysis reveals that the shadow of M87* favors the Kerr
scenario and rules out r₂ > 0.48, while the shadow of Sgr A* exhibits a marginal preference
towards the Kerr-Sen scenario (although GR is allowed within 1-&sigma) and rules out r₂ > 1.
Thus, large values of dilaton charge are disfavored for M87* and Sgr A* and this result
holds good irrespective of the inhomogeneous plasma environment. In fact, the presence
of plasma further constrains the allowed parameter space of r₂ and within the observed
1-&sigma interval, the present data cannot distinguish between the Kerr and the Kerr-Sen black
holes with mild dilaton charges. Moreover, the shadows of M87* and Sgr A* rule out very
dense inhomogeneous plasma environments surrounding these objects and hence, black holes
with less dense plasma environments seem to be good sites to detect signatures of dilaton
charge. These findings not only underscore the importance of considering plasma effects in
shadow related studies but also provide a pathway for refining constraints on alternative
gravitational theories using black hole observations.