It seems like, if one were an evil web developer, one could thwart blockers like these with a few nasty tricks. Server-side: on each request of a resource, salt all linked resource URIs and encrypt (after the host name) before sending the final document to the client. The resources can still be fetched, because the server can decrypt any requests, but the client has no way of knowing whether a particular resource is…
And in response, blockers could then create lists of hashes of ads (pictures) instead of URIs and block those ?
Regardless, it's easy enough to salt the ads for each request, thwarting hash checks.