This simple process utilizing Topaz filters will produce a screen-print graphic poster effect. This was created in Photoshop CS4 and should work in Photoshop CS2/3 and Elements 6, 7 or 8.
I worked with an image that was 1008 x 672 pixels to allow for a large actual-size pixel viewing on this forum. For larger or smaller files different filter settings would need to be applied to achieve the same effect, but the process would remain the same.
AFTER
BEFORE
Graphic Poster Effect:
Start with your image on one "Base" layer
Duplicate the base layer and on this second (top) layer apply Filter menu > Topaz Labs > Simplify > Wood_Carving; click OK on the default settings
On this same layer: Filter menu > Topaz Labs > Clean > CrispStyle, set Strength to 2 > Click OK
Now, we'll change the "Wood Carving" effect from black to a visually softer brown: In your Foreground Color selector (near the bottom of your toolbar) select a medium brown; I used RGB R=133, G=101, B=108
We're still working on the "Wood Carving" layer: Image menu > Adjustments > Hue/Saturation > Select "Colorize" option (notice the black "Wood Carving" effect changed to the Foreground Color brown) > Toggle Saturation and Lightness sliders to desired shade of brown (not too light), click OK
Continuing on the "Wood Carving" layer, set the layer Blending Mode to Darker Color and the layer opacity to 70%
Click on the "Base" layer to activate it: Filter menu > Topaz Labs > Simplify > BuzSim > Set Simplify Size to .20, Click OK and voila! A graphic print
NOTE: On this image the sky looked unnatural, so I carefully selected and deleted it from both layers. I created a layer below my "Base" layer and applied a gradient of two blues, which I feel makes the image look even more like a serigraph (screen print).