Re: how do i achieve this effect on photoshop
There may be other ways to get that effect but it was probably done using the Cutout filter (Filter>Artistic>Cutout) as Johnathan has mentioned. Go to the Planet Photoshop website at:
http://www.planetphotoshop.com/ and watch the "Stylized Comic Book Effect" tutorial (right side of window under Tutorials). That should be helpful.
