If your images are already sized and size is critical:
Open the image
Create guidelines - one each at the top, bottom, left and right
The rounded rectangle tool Anman mentions is under the text tool. Click and hold it to select the rounded rectangle.
Use the guidelines to draw a perfect rounded rectangle. Your photograph will be "hidden" by whatever color you have set as your foreground.
Open your layers palette and Ctrl-Click on the ICON of the shape. You will see the dancing ants.
Highlight the background layer.
Ctrl-J (make new layer from selection)
Delete the background layer and the shape layer. You now have your image as its own layer with the rounded corners.
It's pretty simple to write an action to do this for you, you could make it one-click simple to create rounded corners for any image.