CSS Inset Borders

I would recomnend using box-sizing.

*{
  -webkit-box-sizing:border-box;
  -moz-box-sizing:border-box;
  -ms-box-sizing:border-box;
  box-sizing:border-box;
}

#bar{
  border: 10px solid green;
  }

You could use box-shadow, possibly:

#something {
    background: transparent url(https://i.stack.imgur.com/RL5UH.png) 50% 50% no-repeat;
    min-width: 300px;
    min-height: 300px;
    box-shadow: inset 0 0 10px #0f0;
}

#something {
  background: transparent url(https://i.stack.imgur.com/RL5UH.png) 50% 50% no-repeat;
  min-width: 300px;
  min-height: 300px;
  box-shadow: inset 0 0 10px #0f0;
}

<div id="something"></div>

This has the advantage that it will overlay the background-image of the div, but it is, of course, blurred (as you'd expect from the box-shadow property). To build up the density of the shadow you can add additional shadows of course:

#something {
    background: transparent url(https://i.stack.imgur.com/RL5UH.png) 50% 50% no-repeat;
    min-width: 300px;
    min-height: 300px;
    box-shadow: inset 0 0 20px #0f0, inset 0 0 20px #0f0, inset 0 0 20px #0f0;
}

#something {
  background: transparent url(https://i.stack.imgur.com/RL5UH.png) 50% 50% no-repeat;
  min-width: 300px;
  min-height: 300px;
  box-shadow: inset 0 0 20px #0f0, inset 0 0 20px #0f0, inset 0 0 20px #0f0;
}

<div id="something"></div>

Edited because I realised that I'm an idiot, and forgot to offer the simplest solution first, which is using an otherwise-empty child element to apply the borders over the background:

#something {
  background: transparent url(https://i.stack.imgur.com/RL5UH.png) 50% 50% no-repeat;
  min-width: 300px;
  min-height: 300px;
  padding: 0;
  position: relative;
}
#something div {
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  border: 10px solid rgba(0, 255, 0, 0.6);
}

<div id="something">
  <div></div>
</div>

Edited after @CoryDanielson's comment, below:

jsfiddle.net/dPcDu/2 you can add a 4th px parameter for the box-shadow that does the spread and will more easily reflect his images.

#something {
  background: transparent url(https://i.stack.imgur.com/RL5UH.png) 50% 50% no-repeat;
  min-width: 300px;
  min-height: 300px;
  box-shadow: inset 0 0 0 10px rgba(0, 255, 0, 0.5);
}

<div id="something"></div>