Replace Webex Initial

In Webex Meetings, if you dont have a picture, you are represented by a circle with your initials. You can replace this with an image as well, but it still fits within the same circle.  My goal is to replace this circle with a snap chat filter.  It needs to be the same size as the circle and it is there I am having issues.  I have tried trial and error to create a .png that is perfectly centered and the right size, and I guess I eventually get it, but can anyone think of easier way?  Also, I need the rest of the filer to be white, and it just seems to jump around to where I'll see a white box cover most of the screen, but on the edges, see my camera. When I try to resize te background to fiill everything, it resets back to a smaller area, and I can;t figure out why.

Any help would be appreciated