How does a trained classifier label a picture by its own? Let's say I have a training set, train it using the LDA classifier and it shows me the accuracy/similarities. How do I use this to apply on the picture I want to label?
You have to do that in separate steps afterwards. Your classifier should give you some kinds of instructions for classifying pixels, for example, pixels darker than 130 are background, and pixels brighter than 130 are foreground.