First thing's first: Every image will be rectangular. Even if you cannot see the shape because it's transparent, it's still there.

As mbartelsm said, bmp does not have a provision for transparency, nor does jpeg. If you want the edges to be invisible, you should use png or tiff. You could also conceivably use gif, but png is superior in almost every way (the single exception being animation). And, as far as I can tell, Paint does not have the ability to designate a transparent color, even if you're using a file format that supports it. The transparent selection tool might do what you need, but what you'd have to do would be to select your image, copy it to the clipboard, then paste it into your target application. I'm not sure that CC3 will take data from the clipboard that way, though. I can't test it for you because I don't have CC3 installed at the moment.

The suggestion of downloading the Gimp is an excellent one. It's a free open-source alternative to Photoshop, and it's very well-supported by both the Gimp community and by this one.