I just built (most) of my first skin.  Skinning is a pretty cool technological approach!

I have a 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?

I don't plan to sell it, but I do plan to give it away.  It's oriented towards a club I'm in, and they have branches all over.  So, I am also interested in making things as easy as possible for the skin installer - the user, not the program:), as not all of them will be very technical.

I would place the images in a module folder and reference them all there.

When you package your module use a parent folder  that all your modules get installed into, and inside that folder have an /images folder that all your modules, skins, containers, etc. can reference.

I name this parent folder with my company name, so under /DesktopModules/Snapsis/Images I will have all the shared images.

If I install create another module it will be installed in the same company folder so then I might have /DesktopModules/Snapsis/PageBlaster.

From the folder of the PageBlaster module the relative path to my shared images folder would be src="<%=ModulePath%>../images/shared-image.gif"

For Skins, the folder path is either /Portals/_default/[Skins or Containers] or /Portals/nn/[Skins or Containers] (where nn is portal number) which means they are at the same "depth" so I can find the find the shared images folder from the skin by going back three folders like so:

From any Skin the the relative path to my shared images folder would be src="<%=SkinPath%>../../../DesktopModules/Snapsis/Images/shared-image.gif"

In a .css file from the Skin or Container you are automatically referencing from the physical location of the css file so you would just need something like:
background-url:(../../../DesktopModules/Snapsis/Images/shared-image.gif);

Or if you are not running in a virtual folder you could also just always reference from the root of your website with src="/DesktopModules/Snapsis/Images/shared-image.gif"