You can make a picture look more like a poster by limiting the number of greys.

1. Select the cropping tool from the tool bar, and then select the picture whose image you want to fine-tune.

2. Choose Image Control from the Edit menu.

3. Type the number of greys you want in the Grays text box, and click Posterize. You can have from 2 to 128 greys in a document.

4. Click Apply to see the results.You can make further changes, or you can undo all changes by clicking Normal and then Apply.

5. Click OK when you're done.