= Solution
The <method of images> replaces a conducting boundary by fictitious <point charges> placed outside the physical region being solved. Choose their positions and strengths so that the resulting <electric potential> has the required constant value on each conductor. The <uniqueness theorem for the electrostatic potential> then guarantees that this solution equals the physical solution in the allowed region, provided the other boundary conditions are also satisfied.
For a plane held at zero <electric potential>, a real charge $q$ a height $d$ above the plane has an image $-q$ the same distance below it. The pair's <electric potential> vanishes everywhere on the plane. Its tangential <electric field> consequently vanishes there, as required in <electrostatic equilibrium>. The image is a mathematical representation of the induced <surface charge>, not a physical charge placed inside the conductor. Forces on the real charge can be computed from the image field, but total physical <electrostatic energy> must account for the conductor's induced response rather than simply counting a full real-plus-image pair energy.
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