If you want to learn image processing/computer vision, i suggest you to read some basic books on digital image processing & numerical methods. for example Digital Image Processing by Rafael C is a good start. But it will take time to learn things also to implement it in C or C++.. So for that you can use tools like Wolfram mathematica or Matlab.It will help you to save your time in understanding the concepts.( or make a library of your own , whenever you learn some thing new)
OpenCV is a good library,But i suggest you to learn basics first.
For learning Frequency domain analysis http://homepages.inf.ed.ac.uk/rbf/HIPR2/fourier.htm
Hopes this helps.