code: public @NotNull ImageAssert isEqualTo(@Nullable BufferedImage expected, @NotNull Threshold threshold) {
      checkNotNull(threshold);
      if (areEqual(actual, expected)) {
        return this;
      }
      failIfNull(expected);
      failIfNotEqual(sizeOf(actual), sizeOf(expected));
      failIfNotEqualColor(checkNotNull(expected), threshold);
      return this;
    }
docs: Verifies that the actual image is equal to the given one. Two images are equal if:
  <ol>
  <li>they have the same size</li>
  <li>the difference between the RGB values of the color of each pixel is less than or equal to the given threshold</li>
  </ol>
  
  @param expected  the given image to compare the actual image to.
  @param threshold the threshold to use to decide if the color of two pixels are similar: two pixels that are
  identical to the human eye may still have slightly different color values. For example, by using a
  threshold of 1 we can indicate that a blue value of 60 is similar to a blue value of 61.
  @return this assertion object.
  @throws NullPointerException if {@code threshold} is {@code null}.
  @throws AssertionError       if the actual image is not equal to the given one.
  @since 1.1