1. Open the Style Sheet dialog.
2. Type the following code what's already there:
Note
I've posted the code at http://www.officeliveguide.com/customstyles.aspx if you want to just copy and paste it.
#IWS_WH_Elem_Content img {border: 4px solid #cccccc; padding: 5px;}
3. Save your work and preview your website. Every picture on your site should now have a border.
4. Close the preview window and return to Site Designer.
You styled the pictures on your website to display a pretty border around them. Mind you, the style adds a border to all pictures on the site, not just one.
Note
What if I want a border around only some of the pictures?
With this technique, it's all-or-nothing. To display or remove a border around a single picture, you'll need to ability to manipulate styles on that picture. Unfortunately, Office Live Small Business doesn't offer you the ability to do so.
There certainly are ways to achieve that effect if push...