Hi,
the way you have it set up right now it is not really possible as the images are resized inside a box, so the border-radius cannot be applied correctly.
You would need to first change how the images fit inside the available space, to do that edit the Classifieds List block on your homepage from the wp-admin / Pages panel, click on the Classifieds List, then in the right sidbear go to to the “Grid View Options” section and in the field “Image Fit” select “Cover” and save the page.
Once you do that you can go to the wp-admin / Appearance / Customize / Additional CSS panel and add there the below code
.home .wpa-picture-grid img {
border-radius: 15px !important;
}
That said please note the “Cover” image fit stretches the image to fill all available space, so it will work best (or maybe only) with the images that have (in your case) size 305×224 otherwise they will show stretched.
