pw wrote:
> My question is, after accomplishing this with a development board, which
> low-cost fpga could I buy, and what is needed for a working serial
> interface between the microcontroller and fpga?

Did you already complete this task on a development board? After doing 
so, you should be able (if not yet, you will be able afterwards) to 
answer some of the following questions:

- Is the resulting system fast enough for your needs? (You interface 
from the phone to a microcontroller, maybe through USB 2 or 3, and from 
there you want to have a serial interface to your FPGA, maybe LVDS or 
multiple LVDS. So the data path will be quite busy if you want 
reasonable fast navigation).

- What complexitiy are the algorithms you are using on the FPGA?

- How many pictures you want to process in what time?

- Is the memory on the FPGA sufficient for your needs or do you require 
intermediate storage close to the FPGA? What kind of stroage?

- What feedback information do you want to provide to which instance 
(controller, smart phone, any other)?

- What power dissipation requirements do you have (handheld...)?

From answering these questions (and for sure a couple more will occur), 
the integration from the development board functional model to your 
desired system will lose some degrees of freedom...
...however to my opinion, there will be a lot more limiting factors than 
just "low cost", at least for a prototype.