You can to add a border to a single image

<a href="#"><img style="border 5px solid blue" src="image.gif" width="100" height="50" alt="image">

You can set the colour and the size of the border as desired.

There are also a number of alternatives to the solid border and they are as follows:

Here is an example of a solid green border that has been given a margin of 5px and has the border set to 2 px dashed.

img {       margin: 5px;       border: 2px dashed #00FF99; }