Yes, you can use those formats in Rh9.
"All" browsers - Who knows? The ones you cite should be fine unless you have resized the images within Rh. That does not mean they will not be OK, just that you need to check.
I cannot think of any reason not to use PNGs. Can anyone else give a reason?
See www.grainge.org for RoboHelp and Authoring tips
The only issues I have had with the PNG format is with older versions of Internet Explorer - just be sure you are not supporting old version of IE. I've been bitten by a few EU countries using older versions of IE.
You may find this helpful 'Web Browser Support for PNG'
Europe, Middle East and Africa