How to posterise your images in Photoshop.
Whether your original image is a PNG or JPEG, full colour or black and white, just follow these steps:
1. Upload the file.
Select the image you want to posterise in Photoshop.
2. Turn your photo into a smart object.
Create a Smart Filter. Select Filter from the top menu and click Convert for Smart Filters. You can add, remove, adjust or hide Smart Filters, so any edits you make are non-destructive. (This way you can always go back to the original photo and start again.)
In the top menu, select Image › Adjustments › Posterise.
4. Select the level of posterisation.
A slider window will pop up, where you can select a number between 1 and 255. This describes the number of colours that will appear in each of the colour channels. Move the slider to the left to decrease the number of colours and achieve the posterisation effect. The human eye tends not to register changes in the numbers above 35, so try selecting a number between 1 and 35.