Well, like I said -- don't use pages
as your 404 Error Documents,
use the image you want to use as advertisement as 404 Error Document.
ErrorDocument 404 /images/advertisment.jpg
I haven't tested this yet, but in theory this should mean Apache returns
the image when a 404 is encountered. And that would mean that those
red X's (which are 404's) now return your advertisment image instead.
Before I wrote the stuff above, I hadn't tested this yet, but since I need
it as well for my own service as well, I've now tested it -- works fine:
Feel free to make up your own URLs to append to the domain name to test it futher.