There are programs to do this and you can use photoshop, powerpoint or any other number of programs to accomplish this.
This is a really good point, and proponents of the original proposal should try to articulate what would be added to our shared tree by providing this function that can't be obtained in other applications. As you say, there are already many widely available programs that can alter images.
In my situation, one of my favorite features about WikiTree is being able to work on family history on three desktop computers I have access to, each with a different operating system and browser. Because WikiTree provides an OS-independent (and mobile-accessible) user interface for key genealogy functions like connecting ancestors as relatives and attributing information to sources. All my work is rendered machine- and human-readable by WikiTree without having to be manually converted.
However, because I regularly use different hardware to access WikiTree, I am unlikely to install and learn image-editing software on any of the computers, much less on all of them. And even if I did, it would require some amount of fiddling to configure all three to produce nice-looking watermarks on images I might upload.
So the result is that I am unlikely to watermark any images I upload, even though I could, if I invested the time and effort into learning a new application on each of the three computers I use.
But if WikiTree decided to offer a watermarking function on image uploads, I would probably use it. And, as more users engage with WikiTree on mobile devices, it would be easier for them to upload watermarked images without installing an image editor on their phones.
The value of WikiTree volunteers watermarking their image uploads is probably, at best, an unknown. The original proposal focuses on the advertising benefit to WikiTree. I think there's also historical value to watermarks. The idea strikes me a rather low priority compared to other issues facing WikiTree. But perhaps it can be easily implemented without slowing progress on more crucial matters. If it was, I would use it to add information to our shared tree that will not be added otherwise.