Hi Alison,
After selecting the rounded rectangle or other shape, look up at the bar at the top under the words File, Edit, Image, Layer. Under the words Edit and Image are three boxes: Shape, Path, Fill Pixels. If you click on the shape icon it fills with the color that is active. You want the middle icon with the pen tip visible. That is the path icon and the line you have drawn is a path. When you use that you automatically get a shape that is not filled. The path would then be converted to a selection by pressing CTRL and Enter on the PC. Yes, you could convert the selection back to a path. Select the path palette and down at the bottom next to the new layer icon is a round icon with lines coming off the top. Click on that and you convert your selection back to a path.
dc
If it is not important for the corner radius to match the original picture, you can by corner punches as craft and/or scrapbooking stores. On the high end, you can buy corner cutters that have changable radius tools. However these are costly. Or as several other posters have already mentioned--scissors. Here is a link to corner rounders from the cheap to the expensive.
http://www.factory-express.com/Paper...orner+rounders