Resonant enhancement of high-order peaks in the photo-electron spectrum of argon at 800nm is studied by numerical precision integration of the time-dependent Schrödinger equation in the single-electron approximation. It is shown that electrons in the backscattering region of the spectrum are almost exclusively due to resonances. Wave function analysis shows that there are two types of resonant states: high angular momentum states that stay away from the nucleus and mainly decay by emission of low-energy the ionic core, and are responsible for the enhancement of high-energy photo-electrons.