S1SSEdit is a Sonic 1 Special Stage editor designed to provide a very MS Paint-like interface. It supports many tools for easily drawing shapes, filling areas, and duplicating, flipping, or rotating areas. It does not presently support editing the starting location, as that is stored in a separate file, and will require the addition of a project file system, which will come later. You can download S1SSEdit from SonLVL's Updater, or here. If you're wondering about the numbers on the walls, those are to indicate different varieties of animated blocks. I don't really know how they work. There's an option to turn off the numbers if you want.
Technically I suppose I could, but I don't know how I would implement that. You're perfectly welcome to fork the SonLVL repository and build your own version of the program though.
Since there's a "Export to image" function, what do you think about adding the "Import from a image" function? In the case of my hack especifically, the special stage layout changes according to the difficulty choosen, so this function would be welcome for the lazy people who don't want to duplicate the special stage file to make the harder one. Also, I didn't manage to find if there's any copy/paste function in the editor. EDIT: Very nice job, dude. I'm curious to see what will come in the next updates.
You can just duplicate the layout file itself though? Unless there is some good reason you aren't doing that?
I have been planning some form of image import for both tools, but I don't think it's reasonable to try to import from pictures of a stage, more like it reads an image pixel-by-pixel, and associates each color in the image with a block type. But I'm not entirely sure how to make that work. Also if you can't find the copy/paste feature, have you tried the Select tool?
Would shortcut keys for next/previous block be possible, like SonLVL has? Placing the animated blocks is pretty tedious without it. Maybe even a mode for the line tool that increments the block index by 1 for each block along the length of the line?
It took some work, but I managed to add an option to automatically increment animated wall blocks when drawing with the pencil/line/rectangle/diamond/oval tools (edges only).
Update, now includes a text insertion feature (on the context menu), project support with start position editing (file for GitHub disasm here, you're on your own for 2005), and S1SSEdit now remembers your settings and the files you've recently edited. Additionally, I've upgraded the program from .NET 3.5 to .NET 4.6.2.