how do i convert this image to fit a circle that would print out 6” in diameter? i only have the preinstalled software on my mac.

If you have a new Mac, and it came with Keynote in Applications, then:

1. Choose a white slide, click the existing Text boxes and remove them to a clean slide.
2. View menu : Show Rulers

   If the Ruler scale is in points, you can change that to inches in the Preferences : Rulers panel.

3. Select the circle image.
   1. In the Arrange panel, constrain proportions on 6 inches w x h.
   2. Align Center and Middle.
   3. In the Style panel, you can set the color (or transparency of the circle) as desired.
   4. Click outside of the circle to deselect it.
4. Drag and drop your image onto the slide
   1. In the Arrange panel, click Original Size
   2. in Size, check Constrain proportions
   3. Direct enter width as 4.34 in
   4. Center the image over the circle
5. Select both images, and from the Arrange panel, choose Group, and Lock.
6. Done

Open the file in Preview, select "Adjust Size…" From the "Tools" menu. change the drop down menu to inches. Change height to 6.

go to app store on your mac and find a free image editor there are plenty