I have borders around all my product images. Try this (changing the colors, etc to what you want):

.centerBoxContentsProducts img,.categoryListBoxContents img,#productMainImage,#productAdditionalImages img,.centerBoxContentsNew img,.centerBoxContentsAlsoPurch img,.centerBoxContentsCrossSell img {
border:3px solid #ccccff
}

When you use the FF developer toolbar and select Information > Display ID & Class Details, it will tell you exactly what you need to make the change to in your stylesheet.