Dear FW,
You can thank me later
I give you Sprite Assembly Editor V 0.8.
Here's the rundown:
All "external" files are stored in a folder called deploy in the executable directory. All the sprite assembly info is loaded from a file called SpriteAssemblyInfo.txt. Here's an example one:
#NPCPrettyName,AssemblyPointerAddress (NOT YET IMPLEMENTED),DataStart,DataLength,pathtospritesheet
Crono,242300,1C27B0,26C0,deploy\cronospritesheet.png
Frog,24230C,1C7490,2648,deploy\cronospritesheet.png
Most of that came straight from FW's awesome excel sheet. You can also optionally have a file called SpriteAnimations.txt which gives human readable names to animations like "facing down".
OK, now that you have that set up let's take it for a spin:
1) Start executable
2) Load Rom
3) Select NPC. This list is from SpriteAssemblyInfo.txt
4) The Tilesheet should appear...note this needs to be in a pretty specific form in this version. It needs to be 0xF tiles across and each tiles has to be 32 by 32 pixels.
5) You can now make a "selected" tile by clicking on the tilesheet.
a) Note that now is when you want to select mirror and whatever the hell section is.
6) OK, time to place the tile first select the "super tiile" which is what i called the group of 4 subtiles. You can iterate through by pressing the next/prev buttons (or ctrl + shift + left/right)
7) Once you got the super tile selected you can place the tile with the corresponding buttons (or keyboard shortcuts).

Finally you can move the super tile wherever you want with the buttons (or ctrl + UP/Down/Left/Right).
9) finally, save your work.
It took me about .....20 seconds to make crono a hairy monster. That being said do NOT use this on a rom that you care about.....i seriously just finished working on it and am too tired to test.
Tomorrow i'll try to implement zoom...right now I made the preview real size because i was getting some overlapping bugs when I tried to make the tiles bigger.
--JP