These are flies from the family Ulidiidae. It seems that people did not really bother to give all of them common names, but they are sometimes called picture-winged flies.
Picture-winged flies from the family Ulidiidae on a bracket fungus in Hungary.
They are probably the species Myennis octopunctata but I have only identified them from the Internet.
This particular species seems to like to gather on some bracket fungus. I remember seeing these flies at the same location where the picture was taken decades ago, but I never knew what they were.
Female Ulidiid fly.
After I took these pictures I started looking on the Internet and I have identified them from other pictures — which might not be a good way to identify any insect since there are so many of them.
Male Ulidiid fly.
Further Readings:
Myennis octopunctata on diptera.info.
An article about the somewhat bizarre sexual habits of Ulidiid flies on LiveScience.com.