The influence of image charges on the electron correlation in two-electron spherical quantum dots is investigated. The image charges induced by the dielectric mismatch between the quantum dot and the surrounding medium can induce a transition from volume to surface states, the latter being localized mainly in the self-polarization potential well. Coulomb interaction and correlation effects in these surface states depend strongly upon the ratio of dielectric constants: If ${\ensuremath{\epsilon}}_{\text{dot}}<{\ensuremath{\epsilon}}_{\mathrm{out}}$ the bare electron-electron Coulomb interaction can be screened by the polarizatization terms, then, the kinetic energy dominates, the correlation energy becomes negligible and the electrons behave almost as independent particles. However, if ${\ensuremath{\epsilon}}_{\text{dot}}>{\ensuremath{\epsilon}}_{\mathrm{out}}$ a strongly enhanced angular correlation can lead to the formation of a Wigner-type molecule even in the high electronic density and small dot-size regimes.