Adding borders to images

Hello, I have a custom theme and I am trying to make it where there is a border around the images. I modified the stylesheet. here is ehat it looks like:

/* ———–HTML tags and WP stuff —————- */

a {
color: #000066;
}

a img {
padding:5px;
background:silver;
border:1px solid black;
}

a:visited {
color: #6eaae9;
}

a:hover {
color: #6eaae9;
}

acronym, abbr {
border-bottom: 1px dashed #222;
But the images still have no border does anyone know how I can fix this? I have searched through similar threads and tried what was recommended but nothing works. All help is appreciated.