Eraser Question

I want to seamlessly merge 4 pictures together. I've done this with other programs but when I tried with Corel Photo/Paint I see that I don't have the same options as the other programs. I got used to using create elipse around an object, clicking "Select" then "Invert" which allows me to use the eraser tool to completely erase anything outside of the slected area without loosing anything inside of the elipse, if I were to accidentally go over it. This also gives me a perfect round edge around an object. But so far I haven't found a combination of tools/items that will give me the same effect on Corel Draw. Does anyone know of a way to do this with this program?

Thanks in advance.