i'm actually a developer myself :) ...
well, kinda. i'm studying IT
i developed a game by myself (not anything awsome haha, but it's playable. can entertain for an hour) as a means to study a programming language and have fun at the same time
u are talking about hiring an artist. that would be the ideal, however, until then, i suggest u use this site for free images (in 1 hour u can easily get a decent image for everything:
i have no doubt whatsoever u can get better images on this site than "g" for a goblin for example
another thing u could do is keep the current image (letters) for monsters and just upload the latest playable version u have, but keep the previous version uploaded as an option (and name it something like "dm_optional version with some graphics"?)
the image u showed with walls/floor/player are so beautiful, i would love to play it, even tho there are no animations and new graphics for monsters
please upload your latest playable version (even if it's just for me, i really want to try it out) but keep the older version uploaded for anyone that would prefer it