For the fastest gallery setup, you will not want to use Notepad or an HTML editor to setup the gallery. You will want to use Photoshop or a graphics program.
The best way to do this, and setup the gallery and slideshow at the same time, is to name your pictures by the default names that are used in the template gallery now.
Inside the gallery folder (in most templates) you will see files named for example:
Fgallery1-1.jpg (for the full sized.)
gallery1-1.jpg (for the thumbnail)
You will want to just replace these pictures, or simply name your picture "Fgallery1-1.jpg" and the corresponding thumbnail "gallery1-1.jpg" and then put them in the gallery folder overwriting the current pictures.
This is the best way, because of the complexity of editing the pages themselves.
Bear in mind that every picture in the gallery has two pictures. One for the thumbnail and one for the full sized picture.
You can edit the pages to add your pictures if you want to use your own names, this however will mean you will also need to edit the slideshow as well.
IMPORTANT NOTE: Hosting areas are case sensitive. Be sure you name your files to match what is setup already. For example, be sure you save gallery picture number one as "Fgallery1-1.jpg" and not as "Fgallery1-1.JPG". Not being matched may work fine on your local drive but once you upload the template you pictures may not show up.