There's your problem.
I outlined exactly the way to name the folder and the file in the tutorial. Make the folder name "b29_1"
Then....
Make the actual file "b291.bmp" (no underscore!)
You save the working files, the layered files with _1, _2, etc.... But when you export them you need to name them to over-write that default bitmap. The game is only looking for that original file name on the finished skin product.
EDIT: to clarify, if you're saving a .PSD file or whatever GIMP uses, you add the _1 and whatnot to the name. If you're saving as an 8-bit 256-color bitmap, you save as the original BMP name. Copy those defaults somewhere else if you want to get to them again later for reference. I often have them as the lowest working layer in my graphics file so I always have that layer to reference back to instantly.