OK, there are some Terragen related galleries and some of them are pretty nice places to show your artwork. I don't think that a gallery would slow down this forum. Take a look at Ashundar or the like. Everything is pretty fast there. Regarding chewing up space: This isn't really a problem, is it? And, by the way, animations chew up much more hd-space than images. Harddisks are pretty cheap, even I have a 250Gig-HD in my pc. So, space isn't a problem. To avoid "rubbish", I would call it beginners work, a simple structure with beginners, medium, advanced, pro or so would give you a hint how to rate the image you are viewing.
The education programm idea is worth a thought by Planetside. There are many ways to provide such educational informations: Day by day lessons on the webpage, video tutorials, .pdf-files for download, collections of user-tutorials (would need a standard layout) or a printed (on demand) book. TG2 could be bundled with a beginner tutorial and teachers equipment (dvd with animations, greetings from PLanetside team; images and the like) for school classes with a discount. This works very well, as we have seen in the 90s by Apples school programme.