I create these by generating a PDF per layer from the gerbers, then importing them into photoshop, layering, flood filling, cropping, etc. All this is at 1200 or 2400 DPI so the end product is nicely zoomable.
The output looks nice, and since it is created from the gerbers, it is a "perfect" check of the PCB.
The downside is that there is no net/component highlighting, and that it takes about 2 hours to do it.
It would be SO COOL if the PDF output of Layout could be coaxed into outputting an image like this.