Here’s one with an easy fix. If you’ve just installed WordPress on your server and can upload images but WordPress doesn’t let you resize them in the same form, SSH in to your server and do the following:

yum install php-gd

service httpd restart

And you’re done! …At least, as long as you’re using an RHEL-compatible Linux distro. If not, use your package manager of choice, or manually find and load the php-gd extension!