How can I change the icons to an image?

Just enter this custom CSS and replace the icon URL.

.fa-file-pdf-o:before, .fa-file-word-o:before, .fa-print:before {
 content: "" !important;
}
.fa-file-pdf-o {
 background-image: url("https://www.yourdomain.com/print-pdf.png");
}
.fa-file-word-o {
 background-image: url("https://www.yourdomain.com/word-icon.png");
}
.fa-print {
 background-image: url("https://www.yourdomain.com/print-icon.png");
}

.woocommerce-print-products .fa {
 font-family: none;
 display: block;
 background-size: contain;
 width: 50px;
 height: 50px;
 background-repeat: no-repeat;
 border: none;
 float: left;
}