■Nucleusでサムネイルを作成するのに必要なphp-gdをインストールします。

[root@linux ~]# yum install php-gd
Loading "fastestmirror" plugin
Loading "installonlyn" plugin
Setting up Install Process
Setting up repositories
livna                     100% |=========================| 1.1 kB    00:00
core                      100% |=========================| 1.1 kB    00:00
updates                   100% |=========================| 1.2 kB    00:00
extras                    100% |=========================| 1.1 kB    00:00
Loading mirror speeds from cached hostfile
Reading repository metadata in from local files
Excluding Packages in global exclude list
Finished
Parsing package install arguments
Resolving Dependencies
--> Populating transaction set with selected packages. Please wait.
---> Downloading header for php-gd to pack into transaction set.
php-gd-5.1.6-3.i386.rpm   100% |=========================|  15 kB    00:01
---> Package php-gd.i386 0:5.1.6-3 set to be updated
--> Running transaction check
Dependencies Resolved
=============================================================
Package                 Arch       Version          Repository        Size
=============================================================
Installing:
php-gd                  i386       5.1.6-3          core              109 k
Transaction Summary
=============================================================
Install      1 Package(s)
Update       0 Package(s)
Remove       0 Package(s)
Total download size: 109 k
Is this ok [y/N]: y
Downloading Packages:
(1/1): php-gd-5.1.6-3.i38 100% |=========================| 109 kB    00:05
Running Transaction Test
Finished Transaction Test
Transaction Test Succeeded
Running Transaction
Installing: php-gd                       ######################### [1/1]
Installed: php-gd.i386 0:5.1.6-3
Complete!

■httpd を再起動させます

[root@linux ~]# service httpd restart
httpd を停止中:                                            [  OK  ]
httpd を起動中:                                            [  OK  ]
[root@linux ~]#