how do you get this effect with a simple photo?

[greyhavns]

http://www.asicsamerica.com/flash/advertisingBook/07spring/wide_nevermet.pdf

[DDD]

saturate, contrast. Or maybe even play with levels and thresholds. There is allot of ways to achieve this.

[greyhavns]

cause i don't exactly see how this can be achieved with just some colors

[DDD]

I actually just did, if you go into the threshold or levels palletes and play you will actually see what they do. Or press f1 and search on those terms. I dont have PS open so I cant take a swing for you. But maybe someone else can chime in. But the best thing is to play and see what does what.

[iLikePie]

this is almost leaning towards the 300 effect again (search the forum for a few discussions on that) and like DDD said, it really is about contrast and the saturation of the colors.
The lighting of teh photo will make a big difference - in your example you can see how their legs have strong contrasty lighting which gives definition, and while you can (and would need to) enhance it you can't make it if it isn't there to begin with.

[=guinness=]

the harsh shadows look like they come from over sharpening an image with high contrast... play with smart sharpen and go extreme and you can achieve the odd effect, color is another story.