  Image charge effect is a fundamental problem in electrostatics. However, a
proper treatment at the continuum level for many-ion systems, such as
electrolyte solutions or ionic liquids, remains an open theoretical question.
Here, we demonstrate and systematically compare the image charge effects under
metal and dielectric boundary conditions (BCs), based on a renormalized
Gaussian-fluctuation theory. Our calculations for a simple 1:1 symmetric
electrolyte in the point-charge approximation show that the double-layer
structure, capacitance, and interaction forces between like-charged plates
depend strongly on the types of boundaries, even in the weak-coupling regime.
Like-charge attraction is predicted for both metal and dielectric BCs. Finally,
we comment on the effects of a dielectrically-saturated solvent layer on the
metal surface. We provide these results to serve as a baseline for comparison
with more realistic molecular dynamics simulations and experiments.
