Hi Sebastian,
You can try using a GB Image Block instead, and then through the Block settings, make the borders rounded by altering the border-radius: https://docs.generateblocks.com/article/image-overview/#border-radius
If you want to do it through code, add my-image to the class list of the Container Block holding the image, then add this CSS in Additional CSS:
.gb-container.my-image > .gb-inside-container figure img {
border-radius: 135px;
}