Question:

I want some of my images to be used in my containers AND my skins AND at various places in installed modules.

So, what's the easiest way to do this without having to put the same files in three different directories: container, skin and within the portal?

Answer:

Put the files in the portal folder and use:
<img src="<%=PortalSettings.HomeDirectory%>images/myimage.gif">
In you skin / container.

This will render:
<img src="portals/X/images/myimage.gif">
Where X = the portal number

Now the admin can change the images you referenced.

From this post.