image uploading in events

I think their is a bug in event.module, when i am trying to upload an .PNG image into an event it's show an error
"Error: invalid image format image/png."

and when i am trying to upload .JPEG file then it's shows
"Fatal error: Call to undefined function: imagecreatefrompng() in /usr/local/drupal/drupal-4.1.0/modules/event.module on line 335"

I don't know how to fix it ?