If the computer is asking for permission to make changes to the device that tells me you are probably using Windows 10 or above. That is a feature built into Windows security and it will ask that for any program you install from the internet. Darktable is safe and is reliable. I have been using Darktable for the bulk of my editing work for quite a few years now, and I use it to edit hundreds of photos per week.
Darktable has the option to add watermarks, as well as a lot of other features. It is basically a free alternative to Lighroom, and just like Lighroom there are a lot of features, including mutiple ways of doing colour correction, cropping, retouching, etc, which means it will take a while to get used to.
