First and for most, do you want to do facial recognition with single layer perceptron? single layer perceptrons are not a good idea for such classification based applications. And you need to come up with appropriate features such as gabor coefficients(I don't know if gaussian values are okay) for that,compute these features and train a multi-layer perceptron using back -propagation agorithm.
see
http://research.microsoft.com/en-us/um/people/zhang/papers/ijprai.pdf[
^]
As for OpenCv
http://opencv.willowgarage.com/documentation/cpp/neural_networks.html[
^] see these documentations to help you with the implementation of the your neural network.
I assumed you wanted facial recognition and not "facial image in openCV".
Glad to help :)