LinuxMCE merely wants a jpeg or png image of some sort. The output of the Get Video Frame command expects this.
Where LinuxMCE gets this from? doesn't matter. You just have to provide it.
For the Panasonic IP Camera template, this is done by simulating HTTP requests, and grabbing the resulting JPEG image, and passing it back.
Look at the Panasonic IP Camera code for more details, by going into its device template, and selecting Ruby Codes.
As for Plug and Play, if the device is IP based, and has a certain MAC address range, you can put this in decimal form, in the Plug and Play section in the device template. Again, look at the device template for the Panasonic IP Camera template. You can use the convert_mac utility in /usr/pluto/bin to convert hexadecimal MAC addresses to decimal ones.