You can display image or any other HTML element both above and behind the chart or map. Take a look at this rusty stock chart example. As you see, the chart is transparent and background is visible. Setting background for a page or div using background-image style is the most easy way of having something under the chart. So for example, if you want background under your chart only, add background-image style to your chart's div:
<div id="chartdiv" style="background-image:url('bg.jpg')">
You can also add image under the chart, just remember setting it's position and position of your chart's div as "absolute". Another thing - in case you want more of the background to be visible, set opacities of most elements to some less than 1 value. They will be transparent in this case and the background will be visible.
Images above the chart
You can place any HTML object above the chart too. The same, this rusty stock chart example has some old paint left and it is above the graph. If you view the source of this page you will notice that these are simple images with absolute positions:
<img src="overlay3.png" style="position: absolute;left:578px;top: 216px;">
The only thing you should be aware is that mouse events of the chart won't work while hovering over these images. That's why instead of having one big png with transparent areas we added 4 different images - in this case the area which is not reacting to mouse events is the smallest.