This post should display a header with an overlay image, if the theme supports it.

Non-square images can provide some unique styling issues.

This post tests overlay header images.

Overlay filter

You can use it by specifying the opacity (between 0 and 1) of a black overlay like so:

transparent black overlay

excerpt: "This post should [...]"
header:
  overlay_image: /assets/images/mobile_devices_test_farm-android.png
  overlay_filter: 0.5 # same as adding an opacity of 0.5 to a black background
  caption: "Photo credit: [**Testdroid**](https://bitbar.com)"
  cta_label: "More Info"
  cta_url: "https://unsplash.com"

Or if you want to do more fancy things, go full rgba:

transparent red overlay

excerpt: "This post should [...]"
header:
  overlay_image: /assets/images/mobile_devices_test_farm-android.png
  overlay_filter: rgba(255, 0, 0, 0.5)
  caption: "Photo credit: [**Testdroid**](https://bitbar.com)"
  cta_label: "More Info"
  cta_url: "https://unsplash.com"