Calculation of sampling rate and its effect on an Image?

I know that for a given signal, the sampling frequency $F_s$ must be twice or more than maximum frequency of the signal $F_m$. It is easy to understand the concept for a 1D signal. But I don't know how to calculate sampling frequency or Nyquist rate for a 2D image.

Also what is effect of over-sampling and under-sampling in case of image shown below (dimensions $\text{width} \cdot \text{height}=204\cdot226$)?