Hi Lara,
You can have Auctiva add borders with "Image Borders" enabled in Account Settings. That will work for either the [DESCRIPTION_AND_IMAGES] tag or the individual image tags, like [IMAGE1].
If you are using the URL tags or your own weblink, you could use a CSS inline style, e.g.
<img src="http://mydomain/filename.jpg" style="border: solid 3px; border-color:#000000;">
That example adds a 3 pixel black border, which obviously gives you the flexibility of width and a choice for color.
Danno