It is advisable that all 32bit graphics are released in a standard format. This is to ensure compatibility between different kinds of OpenTTD installations, and ease of manual organising and file management, and automatic distribution.
You can download a template archive made by GeekToo at tt-forums. You can also look at an example release.
A tar is supposed to have the following layout.
readme.txt [author information, license, etc.] sources/ [blend files, materials, photos, rendering instructions, etc.] sprites/ sprites/ogfx1_base/ [main sprite set] sprites/ogfxc_arctic/ [sub-arctic sprites] sprites/ogfxh_tropical/ [sub-tropical sprites] sprites/ogfxi_logos/ [logos & graphics] sprites/ogfxt_toyland/ [toyland sprites] sprites/trg1 [symlink to ogfx1_base] sprites/trg1r [symlink to ogfx1_base] sprites/trgc [symlink to ogfxc_arctic] sprites/trgcr [symlink to ogfxc_arctic] sprites/trgh [symlink to ogfxh_tropical] sprites/trghr [symlink to ogfxh_tropical] sprites/trgi [symlink to ogfxi_logos] sprites/trgir [symlink to ogfxi_logos] sprites/trgt [symlink to ogfxt_toyland] sprites/trgtr [symlink to ogfxt_toyland]
Sources will be omitted in packages aimed at players, but they are still needed to fulfil GPL rules.