The directory files/images is not writable, because it does not have the correct permissions set

the problem

• The directory files/images is not writable
• it does not have the correct permissions set

if that was too complicated...

• A directory is a place on the server where files are kept
• The directory called files/images is not writable by the Drupal (web) service
• This is because the pseudo user that the Drupal web service runs as has not been granted the ability to write to that location
• file permissions are what say who can write things to where on any given system

to fix

• Grant write access permission to the Drupal web user to that directory

How you do that depends on several things you didn't describe in your post - such as :

• What platform you've installed it on
• What access you have to the server
• What is the current ownership status of the files/images directory
• How you put the Drupal files there in the first place
• Whether you know how to use the command line (CHMOD)
• Whether you know how to use an FTP program (CHMOD)

The short answer is probably

The long answer is do some research on what file permissions are. Once you find that out, the answer is obvious.

.dan.
How to troubleshoot Drupal | http://www.coders.co.nz/