If you want to share a custom elytra texture on PMC, you can either upload it under resource packs, or under mob skins/capes with a custom download link to a pack. I choose to do it under mob skins/capes because custom capes aren't usable in vanilla Minecraft, but Mojang uses the elytra texture when they give one out to a player, so its my way of making the capes section on planet Minecraft usable and practical, although I think they should just give elytra its own section.

If you want to make your own download link like i did above you can just copy the custom button image and use it on your upload page, then highlight it in the editor and click insert/edit link and put in your external download link. As far as making the pack itself, there are tutorials all over on how to compile a texture pack, and the only content you need in it is your custom elytra.png texture.

Thanks!