1 Minnesota Jones: {walking down the street towards the Vatican} Once past the guards, we'll have to deal with the lock on the library door.
2 Minnesota Jones: If I know Leonardo, there'll be a devious puzzle involved, which will need to be solved to get the combination.
3 Ginny: "Leonardo"? Don't tell us you knew him, as well as Schliemann?
3 Monty: Some sort of Da Vinci code?
4 Prof. Jones: Do we have time for a last supper before we go in?

The backgrounds are all photos I took in Rome. Panel 1 is Piazza della Reppublica, a short walk from Termini Station. Panel 2 is the Castel Sant'Angelo, and in fact that's St Peter's Basilica in the Vatican in the far left corner, pretending to be something in the other direction. Panels 3 and 4 are Piazza Navona. And standing right between Ginny and Minnesota Jones in panel 4 is my wife.

Leonardo da Vinci of course painted The Last Supper. A lot of people (me among them) are surprised to learn that The Last Supper is a mural, almost 5 metres high and 9 metres long, painted on the wall of the refectory of the convent of the church of Santa Maria delle Grazie in Milan. We're so used to seeing this famous image rendered as framed prints that we assume the original is a painting on a smaller and more mobile medium. The church was actually bombed in 1943 by British and American bombers, and partly destroyed. Some of the refectory walls were reduced to rubble, but luckily not the one holding The Last Supper.

There are various conspiracy theories surrounding the depiction of the Last Supper in the painting The Last Supper, mostly concerned with the depiction of the apostle John, who appears bishōnenesque enough for some to claim he is in fact female. Some go so far as to theorise that it isn't John at all, but Mary Magdelene, this theory forming an important role in Dan Brown's novel, The Da Vinci Code.

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2016-11-23 Rerun commentary: I have a large version of the background photo in the second panel online here. Wow, it was taken on film.

Now that I think about it, quite a lot of the backgrounds in various comics are old photos of mine that were taken on film, before I ever got a digital camera. Combined with the flashback sequence to Monty's youth (#1299 to #1314) which I shot entirely on film... and I suspect that Irregular Webcomic! might be in with a good chance of being the webcomic with the most amount of original production done with a film camera rather than digitally.

(I'm not counting things like Darths & Droids, which use movie screencaps, from movies which were shot on film. I'm thinking things which were shot on film by the comic creator.)