using pseudo :after-element to create a triangle shape color overlay

using pseudo :after-element to create a triangle shape color overlay

I am trying to create a shape color overlay with pure CSS like the&nbsp;<a href="https://scontent.faar2-1.fna.fbcdn.net/v/t1.0-9/50748230_10156519753374213_2195254773937078272_n.jpg?_nc_cat=102&amp;_nc_ht=scontent.faar2-1.fna&amp;oh=b240619bd8f3826c3c3e844005ff9c51&amp;oe=5CFA7EC9" target="_blank">image</a>

I am trying to create a shape color overlay with pure CSS like the image

However, I can't quite figure out how to create the triangle. I would assume I have to use CSS pseudo :after element. However, none of the solutions I have tried so far seems to work and is not responsive at all.

How do I recreate a shape color overlay as shown in the image? Thanks in advance!

My code:

    html, body {
      width: 100%;
      height: 100%;
    }

.bg-img {
  width: 100%;
  height: 100%;
  background-blend-mode: multiply;
  background-color: initial;
  background-image: url("http://unsplash.it/1200x800"), linear-gradient(90deg,rgba(43,135,218,0) 50%,rgba(41,196,169,0.61) 50%);
  background-size: cover;
}


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