Image not found --> creates a path that doesn't exists
When you install Galleria, it creates the image style presets it requires, which you can find under /admin/config/media/image-styles.

If you view the images, imagecache will create the image types and store them in the file system under the appropriate direcory so they will not have to be re-created. If your file system does not have the correct permissions, the directories and files cannot be created, and the images cannot be found.

Check your logs for error messages (/admin/reports/dblog). If this is the case you need to make sure that the directory tree has the correct ownership (usually apache.apache) and the correct read.write permissions (664). If you have access to the server, use the chown -R apache.apache * and chmod -R 664 * commands to accomplish this.

Once you have verified the permission, galleria should work correctly.