It is highly recommended (read: optional!) that everyone developing any graphics for OpenTTD in 32 bits follow these guidelines in order to ensure their work is the best quality possible, as easy for the player to acquire as possible, and as easy as possible to keep going in the event the artist disappears from the community.
If you are developing items for the 32bit base set conversion project, there are more rigorous (and mandatory) rules, which you can read about there.
All graphics developed are to include normal, zi2 and zi4 level sprites. There used to be “extra zoom” and “standard zoom” levels - today, as far as the project is concerned, they are all standard zoom levels.
It is expensive performance-wise to rescale graphics on the fly, and in terms of resulting graphics quality, automatic rescaling is the worst option of all. Also, in order to prevent degradation of image quality, it is recommended that all sprites are prepared and rendered separately for all zoom levels. This is to avoid a messy and/or busy look in lower levels of zoom.
The preparation stage may include LOD (level of detail) work, changes to lighting or shadowing settings, etc. More or less manual rescaling in bitmap editing software can be done but extra measures have to be taken in order to avoid quality degradation. Rescaling bitmaps should usually be avoided because it can very easily lead to bad quality graphics.
Standard tar format
Currently, sprites should be released in our standard tar format.
The standard tar format for 32bit graphics releases is a regular tarball file, originally created by GeekToo, with a standardised tree of contents. Using this format ensures the graphics will work at least to some extent on all OpenTTD configurations (DOS base set, Windows base set, OpenGFX base set, unpatched, EZ-patched).
Releases may or may not be immediately accompanied by sources and licensing information, depending on whether they are intended for distribution with the game or just independent NewGRFs. In both cases, it is highly recommended that the tarball be made available at the 32bit graphics development file repository, with both sources and a GPL-compatible license.
For a description of the format, see 32bit standard tar format.
There is an official repository for 32bit graphics content, started by Jupix in early 2009. All content should go there, including playable releases and their source files, works in progress, etc. For normal 32bit graphics content uploading copies to the repository is recommended, for base set items it is mandatory.
For more information on policies and usage, read 32bit Graphics Development File Repository and 32bit Graphics Development File Repository User Manual.
The standard media for public communication between 32bit graphics developers and players is the forum running at tt-forums.net, and more specifically the OpenTTD 32bit graphics development subforum.
The 32bit base set development thread should be used for releasing, developing and discussing works that aim to be included in the 32bit base set. The Works In Progress thread (AKA “Blend thread”) shall be used for everything else; it is assumed items posted there are NewGRFs. For matters of organisation and management there is the Project Organisation thread. Creating new threads is advisable when developing sets or troubleshooting.