Aha! Whiteboards | Edit opacity and overlay
You can edit any image's opacity and add a color overlay from an image's toolbar.
Select your image, then select the Opacity icon from the toolbar.
Select a standard color or click + to select a custom color (you can enter a hex code for a brand color here), or reset the overlay to transparent.
Use the slider to edit the image's opacity.
At full opacity (with the slider all the way to the right), the color overlay will not be visible. As you move the slider to the left to reduce opacity, you will see the color overlay become more vibrant as the image's opacity decreases.