Find the file called: php.ini on your server and follow below steps
With apache2 and php5 installed you need to make three changes in the...
There's lots of info on the net on achieving that, but I found it a bit too scattered, and had to combine instructions from...
How to access Ubuntu from Windows remotely
It sounds like you don't have the GD extension installed. I don't use Ubuntu any longer, but I think it may be named php5-gd,...