All the button templates are found in skin1/buttons. You would need to either edit the master button template (button.tpl) or call your custom gif images from each individual button template. For example, to replace all of your 'Continue' buttons with a custom image, open up skin1/buttons/continue.tpl and replace this:
Code:
{include file="buttons/button.tpl" button_title=$lng.lbl_continue href=$href title=$title style=$style}
with this:
Replace the 'x' with the actual image sizes and replace imagename.gif with the actual name of the image. You will also need to put all of the images in your skin1/images directory.
Repeat for each button template you want to replace.
Good luck!