Paint.NET is an excellent Photoshop-like image editor for Windows. It is available free of cost, but is not open source.
Unattended installation is documented on the product web site. Several switches are supported. We have chose to force activation of automatic updates (stable releases only), and to force Paint.NET as the default editor for supported file types:
Paint.NET.3.5.5.Install.exe /auto CHECKFORUPDATES=1 CHECKFORBETAS=0 JPGPNGBMPEDITOR=1 TGAEDITOR=1 DESKTOPSHORTCUT=0