I wrote my own method used in list_display (admin class), like this:
class MyClassAdmin(admin.ModelAdmin):
list_display = ('my_own_method')
def my_own_method(self, obj):
if [condition]:
return True
else:
return False
but this value is displayed on list as text (True or False), not as default django boolean icons like this:

What should I do to change this?
Change your code to the following:
which can be found in the documentation on
list_display: