permisssions set to 755 - image upload is not possible - in drupal-commons 6x 24.
You have to set files directory permissions to 777 to make it writable by "others". As I understand you set it to 755 and expect to be writable? 755 means all for the owner, read and exec by group and read and exec by others.

EDIT: As MPD said, if the owner of the files is same user as the one apache runs as than files directory doesn't have to be 777. 755 is enough and it is also recommended.