To Rearrange images, simply drag and drop them to the location you want.
To delete images, click on the images to highlight them and then click on Delete Selected Photos to delete ALL highlighted images.
To add or edit an image Caption or Link right-click on the image.
To choose the Gallery Layout click on the Settings Tab and choose from the Layout drop-down list.
For more information on how to use the Gallery and all the settings options check out this KB article