Besides posting them here, I guess for bezels you can try creating a repository similar to @UDb23's one and submit it to @meleu's rpie-art.sh script so it can be installed by users (see https://github.com/meleu/rpie-art).
For boxart, maybe adding them to a scraping site directly (i.e. ScreenScraper.fr or MobyGames) in addition to posting them here.

If you want to use forum, create a topic for your artwork (or 2 for each type of artworks) and update them periodically in case you add new stuff.