Articulate doesn't provide an option for creating your own marker buttons but, there is a way around to get this done:
1) Add a blank marker to the slide
2) Now select the marker and right click it
3) Under Format Shape-Fill Option, set the Transparency of the marker button to 100%
4) Now place your icon below the transparent marker button.
This is how you can use any image of your choice and still use the marker effect.
