The two aren't even equivalent, are they? Recent Windows versions have a "My Pictures" folder, and Macs have a "Documents" folder; seems like you're mixing and matching.
Each operating system has APIs for finding these special folders. You would need to use JNI, the
Java Native Interface, to call OS-specific native code to find the directories (not to mention that finding them at
all is going to require a digitally signed
applet to get around security restrictions -- you realize that, right?)