Mark,

Be careful about putting images in the portals/[portalid] folder.  If you are only developing modules for yourself, then this is an acceptable approach.  If you are creating PAs that will be installed by others, though, this isn't the way to do it.  For one, you can't presume everyone has their portals in the default portals/[portalid] path.  Probably 99% do, but since the location is an option when creating a portal, users may change it.  Also, it's not a good practice to just install stuff in someone else's portal root anyway.  If I purchased your module, it would upset me if you installed stuff in a location other than desktopmodules/[yourmodule].  Also, since files in portals/portalid are typically accessible through the file manager, anyone with upload privs to that folder could change your images.  You might not want that.

Again, though, if you are just developing for yourself, that's not an issue and you can put them whereever you want.