Where Should I Place Image Charges for a Conducting Wall and Charged Sphere?

[the_viewer]

Homework Statement

We have a conducting and grounded wall for $$z<0$$, so $$\Phi=0$$ for $$z<0$$. In front of this wall, we place a homogeneous charged sphere with radius $$R$$ and total charge $$Q$$. The center of the sphere has a distance of $$a$$ to the front of the wall.

http://david.muelheims.googlepages.com/image_charges.png

I need to find the electrostatic potential $$\Phi(\vec{x})$$ for $$z>0$$ with the method of image charges.
I just need the potential outside the sphere. I do not need to determine the potential inside the sphere.

So... Where do I place the image charges?

Homework Equations

All electrostatic equations.

The Attempt at a Solution

I placed a first image charge in the center of the sphere, because a charged sphere acts like a point-charge in it's center. So I can replace the sphere with a single point-charge.
Then I added a second image charge inside the wall with opposite charge $$-Q$$. This second charged is placed exactly symmetrical to the first image charge.
So... if the first charge is placed by $$z=a$$, I have placed the second at $$z=-a$$.

Is this a correct/possible solution for this problem? Or do I need some different approaches here, because the wall has an effect on the charge on the sphere?

 

[G01]

You are going about this the correct way.