What have I done diddly done

So I'm an old fart when it comes to making heightmaps and don't understand that L3DT program thingy. I figured I could just straight up draw my heightmap like in ye olden days, and it works half the time, half the time does not. I went through the tutorials on mapmaking pretty easily and moved from the tutorial map to creating my own folder and map, I copied and pasted the .world file to my new folder (titled Sam's Map) but changed the N++ file to not say 'tut1.png' on the code line, but my own file which was 'samsmap.png'
I scribbled around real quick in GIMP and went into the game and it worked perfectly! No errors or anything
So I get down to start drawing a real map, looked hella cool, and when I get into the game...
error


I have literally no clue what I did but while I was trying to explain the process through screenshots, I magically figured it all out, so yeah yall can disregard this thread now

Spoiler: Screenshot

It says it isn't in greyscale, which is what confuses me! Or at least that's what I gather from the error. I have made sure that the setting is to greyscale when I start the map in GIMP, and when I draw out stupidly simple maps it works fine but when I actually get down to more complex stuff it gives me this error. Anyone know what to do about it?



Edit:

Spoiler: Further process if it will help??

So here's a lovely heightmap, it's called "heck.png"



I go ingame, and it works perfectly! Boy is it a beauty




So like, since this works, obviously drawing a nicer map will work too?? I've changed absolutely nothing other than the image its self, it's still the "heck.png" file and everything

Stunning


I save it and go into the game




(this is the point where shit just magically started working for me so nvm all this lmao)