Ah, you have discovered one of the most annoying things about X-Cart....the buttons! Basically if you want to use your own buttons, there are a few ways of doing it.
1. You can edit the buttons/button.tpl, as this is the default template called by all of the buttons. It consists of a three cell table, with the rounded corners on either side, and the 'content' in the middle. The advantage of this is it will auto-stretch to whatever text you have in the button. You could replace the three cell layout with your single graphic, just be careful that all the text fits properly.
2. You could also just replace the whole line of code for each button in the templates with a simple html call to your button graphic. Something like:
You would, of course, replace the link with whatever the real link is, and you would have to upload the button graphic to skin1/images.
Hope this sheds some light on the subject!