Unable to change the background image in theme
I think Your have 2 configuration problem.


File Permission (in Filesystem) 
ErrorDocument Attribute (in Apache
Webserver)


First, You can solve permission error using 'chmod' command to the sites folder of drupal.

Try this:


  chmod -R 777 directory (777 means allow permission for all)


Second, ErrorDocument handle error can occured from invaild ErrorDocument attribute setting in .htacess or httpd.conf(or apache2.conf) file.

Try this:


check your .htacess file in 'install/' directory. If you can not
check this file, you can try 'ls -al' command in terminal. you have
to find 'ErrorDocument' attribute, and remove or correct it.
check your apache webserver configuration file (httpd.conf or
apache2.conf). If you can check 'ErrorDocument' attribute, you can
try remove this attribute in file. It is temporary, so you must
backup it before try suggested cases.
If you can not solve by case 1 and 2, you can try disable
'AllowOverride' attribute. Try find 'AllowOverride x' attribute and
replace to 'AllowOverride None'. Because, this problem can have
possiblity from .htacess file in parent directories. AllowOverride
attribute can control allow or deny additional configuration in
.htacess file.
You must restart apache service after try this cases. you can try
'service httpd restart' or 'service apache2 restart' command.


Hope this helps you...