On the title screen, click on the "N" in "MARIO PAINT" and the credits will start rolling. When "PROGRAMMER: NORIAKI TERAMOTO" (the second credit) appears, hold down the right-click button and left-click on the "N" to stop the credits. Now, click on Mario to start the game as usual. Now, four 90 degree rotation options for stamps are available in addition to the regular flip options. You can cycle through them by clicking the flip/rotate command in the bottom left.