Are you wanting the IP camera to take a still image when directed, and normally have video for motion detection? If so, I would say the simplest approach is to separate the two, have a video camera you use for your own motion detection, and get a still camera that takes a wired shutter release for the still shots.
Now, a lot of low end cameras don't have support for a wired shutter release. If you don't care about the camera, you could always take off the plastic housing and put wires to fire the shutter via an opto-isolator from the Arduino. Or you could rig up a servo to press the camera's button. Or you could get a Canon camera that supports the CHDK, which allows you to modify the camera's firmware (CHDK Wiki | Fandom).
One approach is just to blindly take photos every n seconds, and then just discard the shots that are the same.
I did run into a site (http://www.diytrailcams.com/) where people talk about this all of the time, and it might be a better place to ask questions.