The permanent solution I recommend is airing out the humidity after
showering and bath about 15 minutes and
taking out the upper rubber of your bathroom window. So you will
surely have enough dry air and the air humidity can come out
permanently. The next would be a permanent heating of the room to
hinder the cooling off of the walls by night.

All seals on surfaces and in the wall will make things worse, if you
get not rid with the moistured air.

Only to open the window some minutes after showering will not be
enough, what you can see at the pic. The region near the window will
cool off, heating air stream will not come enough to these regions
near the wall-ceiling corner and so the wall sucks a lot of
condensate. Painted with an plastified latex coating, the air
humidity comes in, but the wet condensate in the pores never out:

http://www.konrad-fischer-info.de/SCHIM2.JPG

More info on the link:
Mold attack - A
Guide