Have you tried contacting Chiasm, and any other mappers whose work you wish to use? That's always the best bet.
In this case, if you're editing the images, I would say it becomes an especially relevant case. As Terra said, the graphics are not owned by the mappers in the first place. What the mapper might be able to "claim" and get credit for is the work in putting together a map, including details such as synchronizing animated sprites or how they handle parallax backgrounds. Once you start changing things within the image...well, it might not necessarily be to their liking if you credit them while you change something they wouldn't do.
As per your example I notice something odd. I see that you placed the indoor areas (that are normally hidden by the roof or whatever) into the map. And you know, I get the point of it. But when I look really closely I see that the east side of the stone wall blocks of those areas have a line of black pixels on the right of them not seen on the other wall blocks on the original map. Now I see that he outlined each area including the indoor areas when he made them as somewhat separate maps, and that's what you may be pasting in, but the extra border of pixels isn't part of the original game. Suppose Chiasm is a real stickler for pixel-perfection (and I imagine many mappers are). He might not appreciate what could be seen as an error on a map with his name on it, especially when he didn't make the error himself. And it doesn't even have to be an error, but any sort of change. (Also, maybe he wouldn't like that the row of blocks that are horizontally aligned with the entrances still show the roof tiles instead of the wall blocks, which you would see when you're inside, as per the rest of the buildings.)
If that's not clear, imagine a famous piece of artwork like the Mona Lisa. (Not comparing maps to that by any means, but just go along with it for example's sake.) Say you copied it and altered it, like drew a mustache or a hat on it. No matter how well-painted the mustache and/or hat are, you couldn't really say this new painting was by Leonardo da Vinci, could you? A better thing to do would be to say the original image was by him, implying that it has since been altered.
Same in this case. You have the right idea in acknowledging Chiasm, but I wouldn't say he totally made the map once it has been changed. My recommendation is that you do your best to contact him, if you can't, then you'd better leave the map as it is.
I appreciate that you made this topic to ask, as I am all too familiar with people using maps without asking or even acknowledgement.
