How to Do This Kind of Image

[Mariolee]

I feel like an idiot for not knowing, but how do you make an image like this:
http://designrfix.com/wp-content/uploads/2009/11/awesome-t-shirt-designs-39.jpg

Like, how do you make the solid one color effect on a picture? I'm making a design for school, but they don't want shading, so I want to make a photo of something into one flat color like the instrument on the shirt above.

[Crotale]

Threshold function in Photoshop.

[Manuel_Alejandro95]

What I do is to desaturate the image, then I apply a Levels Adjustment layer and play around with the values. Didn't know there existed the fucntion Crotale said.

[Mariolee]

Threshold function in Photoshop.

Thank you!
Also Manuel, that's what I tried to do before, but I wasn't satisfied with the result. :p

[stevencho]

http://www.vgboxart.com/forums/showthread.php?7058-Tutorial-Gears-of-War-2-Effect-(Photoshop)

Similar effect.