Beginner lost

You need to know that FPGAs are blank canvases for you to add your own IP in to. They can do everything you describe, but YOU need to provide all of the code, connectivity etc. They are also NOTHING like writing software.

Let's simplify the thing. A good way to start would be just reading an image file from an input , make some calculations and just provide a numeric output.

You will need to write a design that contains some form of input, the processing and then output it somewhere.

Ok understood.

There may be reference designs with some interfaces, and IP cores are available for common ones (ethernet, PCI, PCIe, RS232) from altera that you can import into your design. Then its a case of connecting it all up to your custom logic.

And what about a FPGA SoC? would it be easier going on that direction?

The SoC gives you a hard ARM processor on the chip. You still need to set it up yourself, put an OS on it etc. You can put a processor on a normal FPGA too, but a NIOS core instead of an ARM. Either way, it will still take some set up and configuration by you.

Yes , I see your point. My question about the SoC option was in the following reason. Having an already configured ARm processor running for example a Linux OS, the basic functions for accessing files, reading its content are already in place. This way the focus will be on setting up the communication betwen the FPGA and the ARM processor and the calculations on the FPGA. The objective is avoid the aditional work on designing staff like handling files with an FPGA.

This does make sense, but you can easily just send data to the FPGA from an fpga and send it back to to the PC quite easily without an ARM/another CPU. The easily option is RS232 communication. Other methods are possible. The good thing about a CPU is that communcation is easier to set up, but it takes up a lot of resources just for coms.

If you don't have FPGA experience in house, it can be quite a significant cost to build it up / learning curve.