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Actually for Robotics you will be needing concepts relating to Computer Vision, while the Gonzalez book mentioned above is inclined towards Image Enhancement, Image Compression, and gives very little mention of Object Recognition or Vision related applications.
I would recommend picking up 'Computer Vision' by Ponce & Forsyth....and ofcourse you can go through the first 4-5 chapters of 'Digital Image Processing WITH MATLAB' by Gonzalez & Woods. The second book is to get you started with some actual programming using Matlab Image Processing Toolbox....but the first one is what you really need.
Finally if you have a good internet connection, you can check out Professor Avideh Zakhor (UC-Berkeley) Video lectures (again these don't cover Computer Vision, which is what you really need - still the first 10 or so lectures cover the concepts of 2D Signal Processing at some length, which should be pretty useful):