The images reference tip above certainly seems to work across the board using Smarty templates where you want an image to appear in the cart - it sorts out the https / http thing no problem.
This is a typical entry in a .tpl file to use an image:
<IMG src="{$ImagesDir}/spacer.gif" width="1" height="1" border="0">
I have to admit to actually using the standard HTML ../ method of referencing images in certain situations (for example forcing the CC images to appear in the 'Payment Methods' section, as outlined earlier in this thread) but wherever templates are used the {$ImagesDir} method is definitely the way to go.