Those are some nice works. Props to you sir ^-^
But for the car, the bowling pin, the tennis/bowling/magic ball and the pencil hat: heres a tip

Don't use gradient shading. Heres an article/tutorial you should read:
http://gas13.ru/v3/tutorials/sywtbapa_gradient_tool.php
There are other ways you can shade these graphics to look better than they are

gradient shading is mostly used for 3D effects in my opinion. They don't look so great on the pencil hat and the car