Here is a link to a site that has the sandman covers as thumbnails: Neil Gaiman's Sandman - (unfortuntly it's a "geocities" site so there is one "pop-up") so you can get a better idea of the style.

If you get a chance to see the "actual" sandman covers you'll see they have a "mixed media" aspect to them. Often objcts are added to the paintings and text is used as a graphical treatment as well.

The technique is not hard. As Merbliss pointed out, it can be achieved with multiple layers (blend modes and masks) but the creative part, the graphical design, the layout, how all the elements work together, THAT is not so easy.

Your best bet is to study the covers and visually disect them. Take notes, jot down your impressions, what is it in the picture that makes it "click".

my2c
:)
retrocity