Button names can be "adjusted" in includes/languages/english/button_names.php

There are TWO defines:

1. For the name of the image
2. For the image's ALT TEXT in the SRC tag.

Generally, it is not necessary to fool about with these, unless you want to change your image formats. We sometimes use .png images for buttons, because of the "technical" advantages in this image format (transparent backgrounds being better rendered than they do in GIFS).