Hi,
those are not images, those are generated with CSS and PHP.
This css controls the looks of it
.woocommerce span.onsale, .woocommerce .wc-new-badge, .woocommerce .out-of-stock-badge, .woocommerce .free-badge {
background: #ef3f32;
color: #fff;
text-shadow: none;
-webkit-box-shadow: none;
-moz-box-shadow: none;
box-shadow: none;
white-space: nowrap;
overflow: hidden;
position: absolute;
text-align: center;
padding: 4px;
width: 120px;
min-width: auto!important;
height: 26px!important;
min-height: 26px!important;
max-height: 26px!important;
margin: 0!important;
border-radius: 0;
right: -31px!important;
top: 16px!important;
left: auto;
z-index: 1;
text-transform: uppercase;
font-size: 11px;
line-height: 28px;
font-weight: bold;
transform: rotate(45deg);
-ms-transform: rotate(45deg);
-webkit-transform: rotate(45deg);
}
Feel free to paste it to theme options > general > custom css and modify it according to your liking.
The php code to modify is in the file product-image.php
Cheers mate.