I assume you know how to make an application in PIC to integrate the required software module to support the corresponding hardware
1) You can interface an IR sensor to detect motion simply. Select the sensor and look through its data sheet about how to interface it with a microcontroller.
2) Camera Sensors usually support an I2C bus interface so you can controller the register interface through that. For data, it is usually a 10-bit parallel output. Check the sensor datasheet for your selected sensor and you'll be good.
3) It will be your firmware in PIC that will monitor for interrupt from IR sensor and from there it will signal appropriately the Camera controller tasks to take the picutre.
4) If you take high quality or many pictures, you'll need to attach some kind of storage device (e.g. SDcard) with PIC as well. You may also get away with RAM only depending upon your application requirements. OR you can combine it with step 5)
5) You can use USB to send image from the PIC to PC. If this is the case you can avoid storing the picture on-board and can send it to PC directly. Your PC application would have to be running and commanding to get the photo then.
6) On the PC side you'll need to write the driver for your USB device (i.e. PIC that controls sensors).
7) Once the picture is in PC, there are many ways to send it to mobile (off course depending upon what is supported by mobile). If mobile supports WLAN, then that would be easy to use.
This may also help as the starting point
https://www.8051projects.net/downloads119.html