round status strip button

The easiest way is to use an image format that supports transparency: PNG is a good one. Use your favourite graphics program to make the area outside the circle transparent, and you are done.
 
There are alternatives, like setting the Region property for the button to the required circle, but they are a lot more complex, and can give bad results if you do not keep the code in step with the images.
 
I use the transparent method myself.