Usually Cannon uses Picture Transfer Protocol (PTP) and Media Transfer Protocol (MTP) but I'm not sure you can control your powershot with them. Have a look to Gphoto maybe someone has already done the work. The simplest way with your Arduino is to "push" the shot button with an external device a "meccanic finger" or if you don't care about your camera opening it and find the way to hack the "shot button" circuit.

P.S. You can build a simple intervalometer with some resistors, and a relay/capacitor circuit or with a PUT/capacitor circuit maybe you can integrate it with your camera to make the system more portable.