You can use the install Stylish script here (http://userstyles.org/styles/35433?r=1281726605).

As for the rounded corner, I used the outline and -moz-outline-radius property to have it render ontop of the element. But if you use the border and -moz-border-radius on an element, it'll be render around it.

Here's the visuals:

Border around element:

.image {
border:5px solid lime!important;
-moz-border-radius:9px !important;
}

Versus outline on top of element,

.image {
outline:5px solid lime!important;
-moz-outline-radius:9px !important;
outline-offset:-9px !important;
}

To make the effect, just use the same outline color as the parent background, in this case I use white. And using the outline-offset to cover the edge properly:

.image {
outline:3px solid white !important;
-moz-outline-radius:7px !important;
outline-offset:-1px !important;
}