It's an odd pattern, but faint shapes like that generally indicate something on the sensor. Dust is most common, but that usually looks like a somewhat blurred roundish spot. The branching threads here make me think this might be fungus, which is generally bad news. Prague isn't particularly humid, but perhaps you were in a damp situation at some point.

I say "bad news" because while dust is usually on top of the filter stack which is (permanently) affixed to the top of your sensor, I've heard of fungus getting under/inside the filters.

Some things to check:

• Is the artifact in the same place in every photo, or does it move around? Moving around would be good, here, since fungus wouldn't. (On the other hand, some oddly-shaped environmental particle could also stick.)

• How does the artifact change when you change aperture? In general, anything on the sensor is more sharp with smaller aperture (higher f-numbers). It may even be invisible with faster (smaller f-number) apertures. Why use a small aperture when trying to see sensor dust?

In any case, I would start by cleaning your sensor (What is the best way to clean the sensor on a digital SLR?) to see if that helps.

Hopefully it is just something kind of thread-like. You might want to opt for a professional cleaning — or, maybe this is an excuse for a camera replacement, since that model is over seven years old at this point, and technology has definitely improved all around.