Here is a picture of the tool I use to create the levels. I produced it quickly so the tiles on the main grid are letters rather than the actual tile images. If I can be bothered I'll fix this but at the moment the tool is fine for creating and editing the levels.
The level data is output in a format I can past into my assembler. One byte is used for each tile. This byte holds the type of tile, its vertical position and whether there are other tiles in the same column. It also holds some other details that I'll be keeping to myself :)
The tool is a winform application written using C#.