The posterize effect of Photoshop and other digital graphics programs reduces the number of colours in your images. This effect creates jagged edges between the coloured regions. If you want to smooth those edges, Photoshop offers several approaches to let you do so. One of them involves painting over the jagged edges with the Paintbrush Tool. Instead of painting directly onto the posterized image, however, paint onto a new layer. This will let you protect the posterized layer until you're satisfied with your smoothing efforts.
Click the "File" menu, then click the "Open" command. Use the dialogue box to navigate to a photo or other picture to which you'd like to add a smooth-edged posterize effect. Double-click the file to load it into Photoshop.
Click the "File" menu, select "Save as," then type a new name in the "Name" text box to create a duplicate of the original picture. Doing this safeguards the original.
Click the "Image" menu, then click the "Adjustments" sub-menu. Click the "Posterize" command.
Drag the slider in the dialogue box that appears to the left until the number of colours is between two and eight. Click "OK" to finalise the posterizing process. Notice that the edges of the colour regions in your image are now jagged.
Click the "New layer" icon at the bottom of the Layers palette that looks like a piece of paper. This creates a new layer atop your existing image.
Click the "Eyedropper Tool," which looks like an eyedropper icon in the Tool palette, which loads the current colour for painting from a portion of your image.
Click the eyedropper cursor on one of the colour regions in your image, then click "Paintbrush Tool" from the Tool palette. This lets you paint on your image.
Drag your mouse across the colour region that you sampled with the eyedropper in Step 6. Trace along the edges of the region to smooth those edges with the paintbrush. This action replaces the jagged edges of the posterized image with the smooth edges effect of hand-painted images.
Use the instructions from Steps 7 and 8 to smooth the edges of the remaining colour regions in your image. Finishing this step will complete the smooth-edged posterization process for your image.