I’m posting mostly because the repetition will help me remember it, because I keep getting bitten by this.

Mapnik appears to cache images loaded by ShieldSymbolizers (and, presumably, also for LinePatternSymbolizers, PolygonSymbolizers, PointSymbolizers, and MarkersSymbolizers). This means that if you have a long-running rendering process (like Tirex or mod_tile’s renderd), that process uses an image in its rendering, and you then change the image file, Mapnik will continue rendering with the old image. You’ll have to reload the stylesheet in order to start using the new image.