it should be like so:
condition night switch == on
erase picture1
show picture2: night
exit event
else
erase picture2
show picture1: day
exit event
see that you erase pic 1 when pic 2 is shown, then you erase pic 2 when pic 1 is shown, if you want a picture to be constantly displayed then theres no need to delete it in the same condition, also i would test it withouth the exit event