How can I change the default Documents and Pictures directories to symlinks?
You can do the opposite way round: add a symlink of the defaults Documents and Pictures to your Dropbox folder. 

Full instructions can be found here, but basically, in the Terminal, just change directory to your Dropbox folder, and then create the links there.

cd ~/Dropbox
ln -s ~/Documents
ln -s ~/Pictures


Obviously, this will overwrite the existing Dropbox subfolders. You can either rename those folders, or you can create a new name for the symlink in the ln command:

ln -s ~/Documents symDocuments