Automatic media indexing uses Tracker on the N900 and Metalayer Crawler on the N8x0. Metalayer Crawler does not scan for images. You can however navigate to the Pictures folder in the filesystem and tap and hold on it to open the popup menu where you select "Scan for Media". Manually scanning for media is always possible if you don't want to wait for Tracker/Crawler to discover new stuff, or if you want to quickly index stuff not indexed by those.