Storage of the images in Drupal-Commons 6.25: serious troubles acompanied with CHMOD & CHOWN-issues Where do the image files reside? I have problems with the files, and the permissions. On my site, which runs Drupal-Commons, I cannot upload any image, either to a user or a group.

I was not able to fix it. First of all, regarding the user, my server works with the SETGID-bit enabled, and with the user vhost:vhost.
The bad is that every time I touch the chmod, I lose the setgit-bit; I tried changing it to 2777, and then all gets lost. I lose the style too. 

What can I do? Can I upload the images manually, and store them into /home/vhost/WWW/schulcenter.org/sites/default/files?
setuid and setgid are for execution and not access control.

I can't comment on your exact problem, but when I move a site, I do

sudo find /path/to/site/default/files -type d -exec chmod 755 {} \;
sudo find /path/to/site/default/files -type f -exec chmod 644 {} \;
sudo find /path/to/private -type d -exec chmod 755 {} \;
sudo find /path/to/private -type f -exec chmod 644 {} \;
sudo chown -R apache.apache /path/to/docroot/site/default/files 
sudo chown -R apache.apache /path/to/private 


By default, files are in site/default/files but this location can vary for a few reasons


Admin setting for public files
Admin setting for private files
Whether the files use public or private
Whether you use the default location, or a site alias


and probably some others I can't think of.  I suspect that your write errors will show up in your Apache error_log and you should also see what the status report (admin/reports/status) says.

Note that the private paths/settings are for Drupal 7 only, but the rest applies.