Well, you can Build 'SDL_image' yourself,
... or add the rpmforce repository from
http://dag.wieers.com/rpm/FAQ.php#B
As you can see, the command to do it is
# rpm -Uvh
http://apt.sw.be/redhat/el5/en/i386/...l5.rf.i386.rpm
if you have CentOS 5.x "i386"
Then you can do # yum search SDL, so all SDL can be shown,
and to see that your new repo works.
Next # yum install SDL_image-devel
Good luck.....you now have access to another 8,300 packages
with the new repo !
....