Tileset not working

No, the transparent color is always magenta.  The top left corner's tile will display in the editor's tileset panel, but it will always function as a blank tile - hence there's no particular point to putting anything in it.

No, the transparent color is always magenta.

I'd like to add something. On my computer, MSPaint's default transparency is pure black (0,0,0). To have it become magenta, I have saved a tileset with an alpha channel that makes magenta (255,0,255) transparency. Just thought I'd point that out.

Huh?  You're drawing your tilesets in Paint, then opening them with another program, and turning your magenta into alpha transparency?  I think you need to rephrase that, sir.

Also, black is just what Paint converts transparency to when it opens an image with transparency.  This is to say that MS Paint ruins transparent images.