Dotted Border
Here's another way to put a border to a picture. It looks good and works great on a button.
Step 1: First find a picture you want to put border on. Here's my picture:
Step 2: Then click on
Image > Add borders then choose a white border 2px.
Step 3: Now use the
Preset Shapes Tool, which looks like this:

. Choose the rectangle and
uncheck allboxes (Retain Style, Anti-Alias, Create as Vector). Remember to change the line Style to "Dot".
Step 4: Change the background color to transparent in the color materials before you drag the rectangle from one corner to another. Here's my result: