I want to write an app in java that lets two clients talk via webcam. The way it works is both clients connect to a webcam that takes pictures at a specified frame-rate (20 per second maybe) then reduced the size and resolution, then sends it to the other client via a UDP packet. My question is - should I send every picture in its own Datagram Packet? I've read that they can only hold half a kilobyte at most so should every pic be cut down that much? Or should I have it split up into several Packets?
Are you sure you want to transmit whole images, instead of using an algorithm / codec that transfers only what needs to be updated?
If you choose the second option you can take some ideas from this previous question and a already used and tested library for the purpose. I believe i'd go with VLC java bindings if i had to do it. You should evaluate what is the best codec for your specific purpose (bitrates, quality, etc).
If you nevertheless want to transmit images i'd suggest you break them down into udp datagrams, remember that they should be somehow numbered/tagged so that the client can reconstruct the image as packets come (they won't necessarily come in the same order you send them), also you need to think what the client needs to do when some of the packets fails to arrive (discard the image, request previous packet, etc.).
One last thought, the udp datagram max size might not be the best option as well, your server-client should perhaps implement a algorithm and negotiate the udp frame size depending on the speed of the transmission.