Hello Werner and welcome to the forum.
Two points - first, yes, the specification for silhouettes is 68x16px BMP file, but 76x16px works fine, so that's what I do.
Second - only one of the codes you mention is valid - G200 for the Akrotech/Giles G-200, whatever that is! I use the official ICAO site to check the codes:
http://www.icao.int/publications/DOC8643/Pages/Search.aspxThere are many valid codes that we haven't got any silhouettes for, but that's mainly because the aircraft type doesn't fly anywhere that they can be picked up, or that they don't appear on RadarBox anyway - perhaps because they don't have electrical systems or, at least, mode-S transmitters.
Hope that helps.