You can use acrylic paints to prime the inside of the gourd, using even coats, about two would do it. Then you can go ahead and paint your design or picture. The surface of the inside is a little porous and holds paint well.
The outside skin of the Higuera is different in quality than the insides. While the inside is a little porous, the outside might resist paint a little more. If you are using it in its green skin state, it is better to wait until it is dried and has turned a little brown to apply paint to the outside. I use acrylic paint on the outside also. I typically leave a little of the natural wood to show with a coat of varnish to bring out the rich grandular grains of the Higuera.