It sounds like you have 2 cameras set up through icamsource....your iPhone (via ipCam) and your iSight (on your iMac). In my experience (with Windows), I've had USB cameras (built-in cameras) freeze after using them for any extended amount of time, and I'm pretty sure it was driver issues or power-saving issues. It wasn't an issue that only occurred in icamsource....it happened in programs like Skype too. Because I found USB cameras to be somewhat unreliable, I switched to IP cameras. I have had a static image on one of the IP cameras (in the iCam app) when the camera lost network connectivity.
So for the still shot displayed by your iSight, you might check to make sure that your iMac is not entering sleep mode or somehow disabling the camera to save power. I don't have a Mac, but I'd assume they'd have options similar to a PC as far as that goes. And if drivers are required, make sure they're updated. If the IP camera (your iPhone in this case) is displaying a still image, make sure that it's not losing network connectivity.
For the last bit of your post, I'm a little confused because you're talking about viewing the iPhone's live feed in icamsource, but only seeing a still shot from the iSight camera in iCam. iCam should be displaying 2 camera feeds, if it's successfully connecting to icamsource. Does icamsource display any error messages when connecting to the server? Or does iCam display any error messages (usually it takes around 20-30 seconds for the error message to come up) saying that a source couldn't be connected to...or something like that? I'm not familiar with the Mac version of icamsource, but I assume the options are pretty much the same. In the Windows version, there's an option to enable auto router configuration. You should probably have that enabled if you have more than one camera (based on my experience), and you also need to have uPNP (or the Mac equivalent) enabled in your router. My guess is that you're having some sort of connection/firewall issue if you're only seeing one feed, but you're able to see both feeds in icamsource. If motion events are being recorded from your iPhone, that also leads me to think you have a connection/firewall issue between your iCam device (the iPad) and your home network. Or are all devices on the same network when this issue occurs?