I the css in the head you can try something like this
Code:
<style type="text/css">
img.expando{ /*sample CSS for expando images. Not required but recommended*/
border: 3px solid #527BF5;
vertical-align: top; /*top aligns image, so mouse has less of a change of moving out of image while image is expanding*/
}
</style>
This will give you a border that is 3 pixels wide and has a blue color. You can change the color to what you want using a color picker.
If you have trouble getting it working then post a link to your site so we can take a look.
By the way, you should probably forget border:yes; go check out this instead
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