HEMTT is a modern build tool for Arma modding that includes CLI functionality for converting images to/from PAA, making it excellent for automated workflows. 5. Community Plugins (Paint.NET / Photoshop)
Use a tool like TexView 2 to convert the game's .paa file into a .tga or .png file. Edit: Modify the image in Photoshop, GIMP, or Paint.NET. paa converter