The steps to do this are:
  1. Upload the desired images for the button in the Image library.
  2. Find the class or id that is used in the donation forms for the NEXT, PREVIOUS, CANCEL, and SUBMIT buttons.
  3. Insert CSS lines for each of the buttons similar to the following in CustomStyle.css (referencing your new image):
Button {
background:url(../images/content/pagebuilder/10001.jpg) no-repeat;
cursor:pointer;
width: 100px;
height: 30px;
border: none;
}