I made something similar years ago and i used this: http://www.troi.com/software/grabberplugin.html
And i know this is another option: http://www.openideasinc.com/products/videoraptorfmp/videoraptor_1_0v10.php
I hope it helps
When considering the costs associated with trying to get a webcam to work (plug-in, custom software, ongoing development time), you might consider the just-works option of using an Apple iOS device (e.g. a factory refurb iPod Touch around $200) running FileMaker Go and have it running as a client to your FileMaker Pro 13 hosted database. A container field in FileMaker Go will let you capture an image from the device's camera. The image will be added to the hosted database directly.
This off-the-shelf solution would require you to have hands-on to the iOS device positioned at the picture-taking location (presumably mounted on a fixed stand) instead of having the luxury of being at the end of a USB cable.
I wonder why Fliemaker resist to make this doable from the computers camera...?