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This is a very broad question, and without knowing what sort of applications you're targeting, the resolution and frame rate of your images, etc. it is very difficult to answer.
FPGA programming for real-time video is a very different animal from software programming using libraries such as OpenCV. It sounds like a good first step for you would be to shift to a single-board computer that offers many of the features of the desktop environment you're used to. There are many inexpensive ARM-based boards out there; perhaps one of the best known right now is the Raspberry Pi.
The third step to ensure your image processing board is reliable is to test your board thoroughly before deploying it. You should perform various tests, such as functional tests, stress tests, environmental tests, and performance tests, to verify that your board works as expected and meets your quality standards. You should also use appropriate test equipment, such as oscilloscopes, multimeters, logic analyzers, and image processing software, to measure and analyze your board's output and behavior. You should document your test results and fix any errors or issues that you find.