Either take it with an external camera like a phone or purchase a DS capture card for a bunch of money. Any other way they're taking pictures, and the most realistic/common way, is that they're playing on an emulator on their PC.
WILD WORLD WIFI CONNECTION, NO THIRD PARTY SOFTWARE REQUIRED
Here is an overall guide in video format to show you the process.
This person explains it very well. It worked for me and I've successfully gotten a Friend Code. :) If you see this and are interested in possibly playing WW online...