The first image is called Kangaroo Boxer.
In modern-day Russia, the poster seems to be famous for the same reason it's famous elsewhere in the world: for being the "Joey and Chandler poster"! The choice in using Soviet propaganda and cultural posters such as this one to decorate the walls of the characters of Friends (yep, there were at least two others used to decorate their apartments) has been the topic of discussion on a number of Russian Internet forums and message boards. - Friends Props
It is easy to imagine two in-universe reasons this poster might be in Joey's apartment.
- The image of a boxing kangaroo is funny to Joey, and he likes it.
- The previous owner of the apartment left it there, and Joey never bothered to replace it with anything else.
The second image, which is above Monica's bed, is a blown up program from the 1980 Summer Olympics, which was in Moscow.
The only justification I can find for this poster is a line from 'Hooked on Houses' describing Greg Grande's work as set designer.
When it came to Monica’s apartment, Grande wanted to develop “a whole new kind of eclectic taste with a flea-market, whimsical, and anything-goes style.”