Sadly, I don't have the skills to explain how Graphics for SMW works, but MM102 made an excellent videos that covers part of it, though it fully requires you to understand how to hack Super Mario World.

• SMW Character Creation Tutorial (Video URL)
• SMW character creation Aseprite script (Video URL)
• SNES GFX Tools for aseprite (Video URL)

The Adjuster will allow you create Graphics Packs with ease and supports the output from MM102's Aseprite script.

Any futher assistance will be pointed towards SMW Central, as they will provide better assistance than me.

Hosting an index for Graphics Packs similar to LTTP's is unlikely to happen as it may be super annoying to maintain.