Blog Posts Element – Remove post thumbnail border

I am using the Blog Posts element and the post thumbnails have a small black mark in each corner. I believe it is from the post thumbnail being rounded on the corner and the black background is showing. The thumbnail post is the same background color as the page so it would it would look like the two letters would act as a transparent file would.

The problem is with the corners being like that it doesn’t look as good.

How do I fix this. I have searched all over without any luck. Do I make the thumbnail square so it covers the corners or do I make the black part transparent? If so how do I do this, I looked for both solutions and could,’t find either.

Thank you very much for your help with this one, I have spent hours trying to find the solution.