>>9736I believe the way you edit those sprites is to just replace the actual files in the game folder.
%ProgramFiles(x86)%\SMBX\graphics\mario
There's both the sprite sheet and the transparency mask to edit. To test you can probably just save your changes and fire up the game/editor. Making a simple test level to run around in should be pretty easy.
As far as structure is concerned, I'm torn between trying to have some general themes/difficulty targets and running it more like a contest/gallery collection where we take every level and slap em into a long hallway. The more restrictions we place the more we raise the bar to contributions, and I'd rather err on the side of too much content than too little.
That being said, if we can get some artist-types to supply some standard issue pone-related sprites, enemies, blocks, and backgrounds that would be excellent. I think having a sweet teaser screenshot for the overworld or a quickie level that demos all the custom assets will do wonders for getting people on board.