permisssions set to 755 - image upload is not possible - in drupal-commons 6x 24. hello and good day hope  you are all well! 

newest update:
Now i have created a second installation with partly other conditions:(haviing permissions 755 with user vhost: www on /home/vhost/WWW/campus-24.com/sites/default The selected file xxxxx could not be uploaded. Only JPEG, PNG and GIF images are allowed. Well this is a new error - i had not this error before - what can i do. Guess that i have a made a big step... what do you think!?

Here the original-thread-start: just installed a drupal -commons version 6x 24:

when i try to upload images i get errors - and after doing some search in the system and the adminstration-controll i fould out: the reported error 

image upload is not possible - in drupal-commons version 6x 24 {released on jan 18th ):  i have done some research and found out that i have: 755 -  in directory sites/default/files 

Allthough i have the permisssions set to 755 - 

what can i do here - just lemme know! 

The directory sites/default/files is not writable
see the path where i i have recogniczed the error 

http://schulcenter.org/?q=admin/build/themes/settings

The directory sites/default/files is not writable

Well what can i do here?!

update: thx alexandru and Mpd - thx for the answers; well everytime i touch the permissions i get  lost.- (i have troubles n issues with permissions and file users. see here https://superuser.com/questions/382792/setgid-bit-always-unset-when-changing-file-permissions  hmmm - well MPD guess that you are right and i am pretty lost -here . what can i do now??
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.