You can use the image import module to import a large number of images into Drupal. The image files need to be in a folder on your server, so you will need FTP access or a more secure form of uploading files to your server such as over SCP or SFTP.
- Create a folder on your server in which you'll place your images to import. 'files/import' within your Drupal folder is a good place. Set this folder name at Administer - Site Configuration - Images.
- Go to Admin - Content Management - Image Import. You'll see a list of the images in the import folder. Select the ones that you want to go to one gallery and select the according gallery you have previously created. Import and just go to Image Galleries and check it out.