Retrieve frequency value of an 2D frequency spectrum
You have a signal in a 2D-frequency domain. You know the maximum frequencies in both directions, I assume. Hence, you know which bandwidth is represented by one pixel and then... just count the pixels from the center to the point of interest along the axes and your done. Depending on your application, taking the Euclidean distance from the center to the peak is also an option, which does end in one instead of two frequencies.

If you would like to do a more sophisticated approach, you could fit a Gaussian distribution (or another function, if you have one in mind that is more appropriate in your case) to the peak. That way you could get a sub-pixel accuracy of the location of the peak.