Editing photographs in paper
As far as I'm concerned, if you're using someone else's photograph and you don't have a clear set of rules stating that you can edit it i would avoid doing so, unless it's a very, very basic thing, like cropping it to hide the cars. 

You could also use the legend box commonly written below the picture to help explain it, for example: "Recent picture of a church constructed on the XII century." 

If the visual aspect of the church is really important to you could try to recreate how it would look when it was recently constructed (maybe pay a designer to do so) and make it your own thing (of course, referencing the original author: "Recreation of what the church of bla bla bla would look like on the XII century. The original picture was taken by XXXXXX and treated using (...)"