Hot electrons of variable energy were injected from a scanning tunneling microscope tip into a Pd/SiO2/Si(100) metal-oxide-semiconductor structure. An analysis of the emerging collector current in the Si substrate, a technique known as ballistic electron emission microscopy, revealed a monotonic barrier height lowering with increasing positive oxide bias, in excellent agreement with a scaled classical image force theory. Calculations using the WKB approximation suggest a negligible contribution to the observed shifts from electrons tunneling through the barrier. From an extrapolation to zero oxide field the Pd-SiO2 barrier height of 4.08 +/- 0.02 eV was deduced. An image-force dielectric constant of 2.74 in between the so-called optical (2.15) and static (3.9) dielectric constant was determined. In order to understand this intermediate value, a theoretical calculation of a retarded image force on the moving electron is carried out for the first time. The calculations yield an image-force dielectric constant of 2.69, that is consistent with the experimentally determined value. This intermediate dielectric constant is evidence for electron-phonon interaction and corresponds to an average dielectric response integrated over the time of progression of the electron in SiO2.