upd нашел интересную темку
http://sourceforge.net/projects/wally/files/ ...
выдрал из кода той софтины:
#ifdef Q_WS_MAC
newFile = adaptPhoto(currentFile.absoluteFilePath(),"PNG",currentPhotoInfo);
QFile scriptFile(_tempStorageDir + "/wally.scpt");
QTextStream scriptText(&scriptFile);
scriptFile.open(QIODevice::WriteOnly);
scriptText << "tell application \"Finder\"" << endl;
scriptText << "set desktop picture to POSIX file \"" << newFile.absoluteFilePath() << "\"" << endl;
scriptText << "end tell" << endl;
scriptFile.close();
args << QDir::toNativeSeparators(_tempStorageDir + "/wally.scpt");
QProcess::execute("osascript",args);
#endif