If the button and shopping cart images appear too Big or Small on the Thesis Theme then read the following:
The Thesis theme specifies the width of the input field in it's CSS which makes the images to have a specific size and causes this issue. You can override this CSS by adding the following to the custom CSS file that doesn't specify the width so the button image size is the actual size of the image:
.custom .format_text input {
border-width:0.067em;
padding:0.2em;
width: auto;
}
So all you have to do is click on the "Custom File Editor" menu from your "Thesis Options" menu and paste the above piece of CSS code.