Error when attempting to upload graphics to Nelo Theme

This is a newbi thing I am sure, but I am using the Nelo theme, I attempted to upload an image using the custom homepage images section, under the Blog Top Header Image Upload And Crop selection, when I click on save thumbnail I get the following error

Fatal error: Call to undefined function imagecreatetruecolor() in /public_html/wp-content/themes/wpmu-nelo/functions.php on line 1184

what maight I have done wrong?