app_icon = QtGui.QIcon()
app_icon.addFile('gui/icons/16x16.png', QtCore.QSize(16, 16))
app_icon.addFile('gui/icons/24x24.png', QtCore.QSize(24, 24))
app_icon.addFile('gui/icons/32x32.png', QtCore.QSize(32, 32))
app_icon.addFile('gui/icons/48x48.png', QtCore.QSize(48, 48))
app_icon.addFile('gui/icons/256x256.png', QtCore.QSize(256, 256))
app.setWindowIcon(app_icon)
PyQt4 set windows taskbar icon