Change shape of blog post thumbnails

Site URL: https://www.postcardsfrompeta.com

I want to change my grid style blog page to show the featured posts/photos reshaped as an arched shape

Any help will be greatly appreciated.

If you use blog summary blocks I can help reshape the items.  I did that for my site. http://Myrandomviews.com/blogsummary

I could possibly change to that layout, how do you shape from there?

To answer my questions and to help future:

Add to design > custom css

/* round image corners blog thumbnails */
.blog-basic-grid .image-wrapper {
  border-radius: 100px 100px 5px 5px;
}

My template Avenue only shows blog posts as a list, not wall or grid like other templates which is way I use summary blocks. This is the code to shape the block item. You can tweak the border radius numbers and make different shapes just as you can with the code you posted. 

.sqs-block-summary-v2 .summary-item {
  background-color: #ffffff;
  border-radius: 70px 15px 70px 15px;

}