Mixins Accept Parameters

Like functions, mixins also accept parameters.
_mixins.scss

// Rounded elements
@mixin roundy($dim, $brdr) {
  width: $dim;
  height: $dim;
  border: $brdr;
  border-radius: 50%;
}

_images.scss

.img-featured {
  @include roundy(165px, 4px solid $white);
  margin-top: 75px;
  position: relative;
  z-index: 100;
  @media (max-width: $break-xs) {
    display: none;
  }
}

Mixins with null

Sass supports a null data type that lets you define optional mixin parameters without creating unnecessary CSS output.
_mixins.scss

// Rounded elements
@mixin roundy($dim: 50px, $brdr: null) {
  width: $dim;
  height: $dim;
  border: $brdr;
  border-radius: 50%;
}

_images.scss

.img-featured {
  @include roundy(165px);
  margin-top: 75px;
  position: relative;
  z-index: 100;
  @media (max-width: $break-xs) {
    display: none;
  }
}

Mixins with Keyword Arguments

Also, remembering the correct order of arguments can be tricky. Sass solves this by allowing you to name the arguments you pass to a mixin, using keyword arguments.
_images.scss

.img-featured {
  @include roundy($brdr: 4px solid $white, $dim: 165px);
  margin-top: 75px;
  position: relative;
  z-index: 100;
  @media (max-width: $break-xs) {
    display: none;
  }
}