Can I use openCL to develop an FPGA Accelerator device for an mac application

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Altera makes 3th way -- from embedded ARMv7 in FPGA, booting from SD-card and running with Linux.

If you may compile kernels from Win/Lin and write its to SD-flash, then may insert big PCIe board to Mac and send control manipulations through ARMs through PCIe, write data arrays to shared memory and read back results from it.

However, I recommend NVIDIA GPUs -- more faster for image processing.

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That would only work for SoC FPGAs that have an ARM core. The majority of high-end FPGA boards only have a non-SoC FPGAs and will need to be connected to a standard x86 host to be usable with OpenCL.

And of course, if you are comparing high-end Nvidia GPUs to low-end SoC FPGAs, the GPU will be much faster for any workload, not just image processing.