What?
Sounds easy but actually if you use an image which has transparent areas, ie. has one color which will be transparent, the transparent pixels will be colored in with the page background color.

Why?
I want an image to display per row as a status marker for 3 different types of results: Success, Failure, Unknown. My images are circles with the background being transparent. I want the first column to display an image based on the status result.

If I simply insert an image, the report would use the background color of the report.
How?
The issue exists if your image has transparent areas but displays the report background color instead:

1. Right-click on the cell you want to put the image in.
2. Insert > Rectangle
3. Set the Rectangle Fill color (I'm using an expression based on the status) to output: Green (for Success), Red (for Fail), Orange (for Unknown).
4. Right-click on the rectangle and select Insert > Image
5. Edit the Image Properties to specify the image to use.

The image can be an expression as well (I embed these and then refer to them in the expression by name). For example: