I have 96 face images, each image has 16 haar values, each of which has many values, and the question is how to use PCA for feature selection in the image? and how do I use it in matlab? because I don't really understand how PCA works
Well, you could try whether PCA finds a linear mapping among the 16 Haar wavelets that expresses most of the variability in your images, but not sure how that would generalize to other images. Added Wavelets as keyword so a colleague more knowledgeable in that can handle any follow-up question.