For those who don't want to do FTP etc.

Let us say the base image of the product is called xyz.jpg

Name the other images of the same product as xyz_01.jpg, xyz_02.jpg, xyz_03.jpg etc. in the sequence in which you want them to appear. For larger image of any image the name should be xyz_LRG.jpg, xyz_01_LRG.jpg etc.
etc.

Now go into the product edit page and upload one image at a time. Make sure that the Base image (xyz.jpg) is uploaded the last. Other images can be loaded in any sequence.

You can load the same image multiple times with no harmful impact. Make sure that you end the uploading process with the base image. Even if you have already loaded the base image earlier you need to upload it again at the end.

CP
http://################################.com