 Unequal charges above a grounded conducting plane

For real point charges $Q$ and $-2Q$ at $(0,b)$ and $(a,b)$ above the plane $y=0$, the image charges are $-Q$ and $+2Q$ at the reflected positions. The image charges enforce constant electric potential on the plane. The physical forces follow by summing ordinary Coulomb's law forces on each real charge; the electrostatic-energy factor of one half for induced charges does not multiply this force. The smaller positive charge can have an upward total force component, even though its own image attracts it downward.