No thumbnails in your wordpress? Read on…
I noticed last night that my wordpress is not generating thumbnails from uploaded images like it’s suppose to. You’ll noticed that the size options (thumbnail, medium, large) are all disabled except for the full size.

After doing a lot of searching, I finally found out that in order for wordpress to generate thumbnails “by default”, you need to have GD library support for your php. It would have been nice if wordpress had documented that better.
The only reference I found was this:
http://wordpress.org/support/topic/235625
To be clear, wordpress does not generate thumbnails by default unless your php instance has GD support.
Install php gd library
For me, installing GD library was easy:
# yum install php-gd
Check for gd
To check if you have GD loaded in php:
# php -m [PHP Modules] bz2 calendar ctype curl date dbase exif ftp gd gettext gmp hash iconv
