Since you mention you're new to this, I should also explain: after you've made
the edits, you can save the file (as a JPG), or you can save the file as a
Fireworks PNG (different than a web 8bit/24bit/32bit PNG), which you can later
open in Fireworks and still be able to edit the ellipse mask.

On the website, you should use a JPG, though, not a Fireworks PNG. Usually I
have a Fireworks PNG on my computer, and a JPG version on my computer which I
upload and use on the website.