Transparent background, with ellipse no fill, but outside of it filled

[ASG_Captivater]

I'm trying to do something which I think should be quite simple but I can't figure out how to do it.

I want to make an image with a transparent background that can be placed over other images. I want the frame around the other images to be round so I insert an ellipse on a transparent background. But then how can I fill the outside of the ellipse so that the only transparent part is the inside of the ellipse?

Do you understand what I mean?

Any tips much appreciated.

I also have photoshop and illustrator if it is easier in one of those.

Thanks!

[ASG_Captivater]

I solved this by painting it in by hand on a new layer. I'm sure there must be a better way but this worked. I'm still interested to learn a new trick if anyone has one. I tried using a mask but it works the other way around. Couldn't figure out how to get it to do what I wanted.