No photo, but yesterday morning, whilst cycling over Putney Bridge in London at 10.45, there was what I thought was an Airbus, either A340 or A380 descending towards Heathrow. I was only seeing it from the rear, which is why I couldn't really tell what type it was.
However, that's not the unusual bit. Inboard from the No. 3 engine on the starboard wing was another engine pod although it was definitely smaller than the working engines. I know airlines do sometimes ferry replaced engines in this way, as about 20 years ago I saw a 747 in a similar configuration, but I thought it was odd that it was clearly a smaller engine.
A bit of a late response, but ...
Some 747s can indeed carry a fifth, non-operational engine, for transport to an unusual location. The engine has to modified slightly for transport, fan blades removed, and covers over some of the inlets, and the aircraft has to have been built to support this load, and only some airlines have ever had this facility.
It's use has fallen away, with increased ease of getting large cargoes to locations, with generally better transport infrastructure to many parts of the world, and the availability of things like the An124 to transport oversized loads, relatively easily.
I've never heard of an Airbus having this facility. It's possible that it was some sort of testbed, since it's not that unusual for aircraft to be modified either with an additional engine pylon, or an existing engine being replaced by a prototype for testing. In essence they're flying wind tunnels!
Obviously these aren't passenger aircraft, so you can do something like replace one of the four engines of a 747 with an engine which could quite possibly be non-functional at take-off and landing, and only operated in flight, when it would be heavily telemetered to onboard data collection equipment. Since they generally won't be flying long distances over water, and so forth, and may not be near the aircrafts maximum payload, missing an engine (or having the additional weight of an extra engine), isn't necessarily a problem.
Assuming there's space on the wing, an additional engine can be fitted, but it's obviously a complex exercise, since the engine has to be fitted somewhere that has the clearance, can take the loading, and the aircraft likely has to be balanced with something acting as a counterweight on the other wing (this could be something as simple as additional fuel).
Another approach is to have the extra engine mounted adjacent to the fuselage on a pylon extending out from there. It's relatively easy to do this, compared to hanging it off of a wing, mechanically, and it has similar characteristics in terms of the airflow, when you're just trying to test an engine in a (sort of) realistic flight configuration.