I used Power Point 2019:

Let´s use the same image:

"Formas" and then "Elipse":



Use the size that you want:



By default the color is blue, let's change it, choose the circle first and you´ll "Formato de forma":



Then use this section:





Choose the color of the sun:



And you´ll see the change:

The "Contorno de forma" and use the color that is already there:



Let´s move to the background and then the sun effect:

Make a square and use the same steps that we use to change the color to the circle:



This option could be important to put the image behind:



After that move the square as you want and put it behind the sun:



Ok, let´s go for the sun effect, select the circle and the choose "Efectos de forma" and "Iluminado":



Then "Opciones de iluminado":



On the new panel select again the color of the sun:



And finally on "tamaño" let´s give 20:



And that´s it now we have the sun effect:



To save the image go to "Archivo":



Then follow this steps:



And save the image using "PNG" file:



After that you can edit the PNG file as you always do.

I hope this help you.