chose FPGA

It really depends on what you are doing with the image, and how you're going to get the image into the FPGA.

What I would do, is make a list of the features you need the board to support, and then try to at least get some idea of the number of FPGA resources you need based on the function you are trying to implement. IE How many Memory blocks, How many multipliers, how many LE's.

The FPGA Resources requirement then double (at least) that should give you a Target FPGA size, then the other feature requirements will give you the board.

Pete