More GIMP goodies this time. This is a GIMP tutorial on how to add Fog to a Photo or render. The tutorial comes with 20 fog overlays included free of charge. Since you have to watch the tutorial anyway, you can check the description for the fog overlay downloads on YouTube. Tutorial

The tutorial also tells you about a GIMP plugin called G'MIC which you can also use to add fog to your pics. There doesn't seem to be a link on the page for that so I include the plug in link for you here: The plugin is also free.

Quick addendum: If you use Krita, you can pick up the G'MIC plugin for Krita at the same page.