I'm going to be honest, but one of the reasons I left was because of the graphics. If graphics don't appeal to players, don't expect them to stay very long. You need a solid team in charge of graphics standards as already mentioned, as well as a team in charge of advertisement. It seems to me that you have very limited staff when it comes to things like this, and my guess is that it's due to too broad of assignments.
Example: if you want to develop a website, don't just take a couple of guys from your graphics team and ZP. Instead, go for quality and hire a team to do it for you. This may seem too farfetched, but it takes work in order to get good results.
Edit: I partially agree with what Imprint said, not completely. It's a big game of cause and effect. The way you run the server reflects on the content, advertisement, and everything in between. I can't go too deep into an explanation because it's such a broad topic.
Another thing, that tileset definitely needs to be updated. Hire a decent graphics team to help with the development of an updated tileset that works well for what your gmap goal will be rather than putting the puzzle pieces in the wrong places and hoping it looks nice.
Yeah... One last thing
Competition. Whether it's against other graal servers, or a completely different company, it's a competition. This may be graal, but you're in control of the way the server is run, and I don't think your superiors would be mad if you drew in more players than the rest of the servers through advertisement, because in the end it's them getting the money. Do whatever it takes to draw in players as long as it doesn't degrade the other servers.
Everone who replied to this post before me and those who will reply are proof that zone has massive potential, because we wouldn't be replying if otherwise. It's only a matter of whether you use that potential for the better.
Lastly, if I were in your position, I'd treat it like a business, not a game. In order to make money, you gotta spend money. Don't go cheap, because quality is key.