I am creating a custom gallery. I require images that are the thumb nail and the big image size. I have custom content type. When i add the image field it allows me to enter the dimensions size. Is there a way for drupal to create 2 images. ie thumb and big image
ImageCache is what you are looking for. Here is a quote about it (from the module's project page):
... allows you to setup presets for image processing. If an ImageCache derivative doesn't exist the web server's rewrite rules will pass the request to Drupal which in turn hands it off to ImageCache to dynamically generate the file.