If you have a new Mac, and it came with Keynote in Applications, then:
- Choose a white slide, click the existing Text boxes and remove them to a clean slide.
- View menu : Show Rulers
If the Ruler scale is in points, you can change that to inches in the Preferences : Rulers panel.
- Select the circle image.
- In the Arrange panel, constrain proportions on 6 inches w x h.
- Align Center and Middle.
- In the Style panel, you can set the color (or transparency of the circle) as desired.
- Click outside of the circle to deselect it.
- Drag and drop your image onto the slide
- In the Arrange panel, click Original Size
- in Size, check Constrain proportions
- Direct enter width as 4.34 in
- Center the image over the circle
- Select both images, and from the Arrange panel, choose Group, and Lock.
- Done