Devilmarkus released a standalone version of CPCPaint. With CPCPaint you can draw images and convert PNG, JPG, BMP and GIF into standard CPC screens. It allows you to create overscan (32k) and standard (16k) screens for the CPC. It also integrates an effect mixer with which you can handle the contrast, RGB values, brightness, etc.
You can download the tool from Devilmarkus’ website.