The term "image charge effect" comes from the image charge method, which is a general way to simplify the calculation of the interaction between a charge and a dielectric interface or a conductor, on the basis of the "uniqueness theorem" [1].
I. image charge effects due to the dielectric interface
The resulting effects of the presence of a dielectric interface are the following:
Imagine a water-oil interface,
1. If a charge is put in the high dielectric phase (water), then the charge experiences a repulsive force from the dielectric boundary.
2. If a charge is put in the low dielectric phase (oil), then the charge experiences an attractive force from the interface.
II. image charge effects due to a conductor surface
Because of the restraint that the electric field lines have to be always perpendicular to the conductor surface, the image charge is always of the opposite sign of the original charge.
III. Applications
The image charge effect can be conveniently used to explain many phenomenas. One of them is the pKa shift of an ionizable probe near the water-oil boundary of the micelles [2].
References:
1. Ken A. Dill and Sarina Bromberg. Molecular Driving Forces. Garland Science, New York and London, 2003, p399-406.
2. Soderman et al. J. Phys. Chem. B. 2006, 110, 3288.