"How much theory for recognizing objects in images?": Some pattern recognition, lots of image processing.
For the most part, it doesn't matter what classifier you use: k nearest neighbor,support vector machine, random forest, neural nets. They'll all give about the same performance. You should have a general idea what they do, but I don't think it's worth the effort to become a "neural net expert". You should know enough pattern recognition so you don't fool yourself (by over-training, for example), and have an idea for how to choose the right features.
Where should you put your effort? Into finding useful features for the object you want to classify. And the more image processing you know the more useful features you'll be able to try. How much do you need to know? Depends on the problem. If you're finding cars in the desert then not so much. Your feature set might be "has long straight lines and is not sand colored". If you're trying to tell American made cars from Japanese then it's harder (unless they are moving, in which case it can't be American).
And what is needed for face recognition (face matching, not just detection), in the same terms? Are the same kind of tools enough for this? (from what I read, it seems so, but so far I couldn't completely believe it)
For the most part, it doesn't matter what classifier you use: k nearest neighbor,support vector machine, random forest, neural nets. They'll all give about the same performance. You should have a general idea what they do, but I don't think it's worth the effort to become a "neural net expert". You should know enough pattern recognition so you don't fool yourself (by over-training, for example), and have an idea for how to choose the right features.
Where should you put your effort? Into finding useful features for the object you want to classify. And the more image processing you know the more useful features you'll be able to try. How much do you need to know? Depends on the problem. If you're finding cars in the desert then not so much. Your feature set might be "has long straight lines and is not sand colored". If you're trying to tell American made cars from Japanese then it's harder (unless they are moving, in which case it can't be American).