Changing the brightness is not the same as changing the opacity. We really need to be able to change the OPACITY of a photo, so that the photo layer be 50% transparent.
That way we can add text or solid color over the photo and the photo won't be so overpowering.

Please add an opacity feature for images, it would be such a huge help!

When you have text and a logo on a bright image, this would be a good solution.

The darken filter is too dark at times and takes away the focus of the image.

You want the right balance between text that is readable and a prominent logo.

The same question is for the video? For the images that I use, I want to adjust its opacity, chnage text formats, add more effects and transitions.

NOTE: I am a premium Spark user, yet unable to customize the slides on my own.

I understand what you are asking for. Here is a workaround.

If you are using the Web version of Post:

1. Click on your text box.
2. Click on "Shape" in the panel on the right.
3. Click the the shape that is 3rd from the top, in the first column. (It's a square with an "abc" in the middle.) This is a "fill canvas shape". Note: If you click on a shape multiple times, it cycles through different options for the text — None, Cutout, Cutout with Outline, then back to None.
4. Click "Done".
5. Click on "Opacity" in the panel on the right, and adjust the opacity to screen back the background image(s).
6. Click on Color in the panel on the right if you need to change the color of the shape behind the text.