A lot of people have been having great ideas, and I'd like to build on that more than anything else.
There is a category on the website Lifehacker called featured desktops (http://lifehacker.com/featured-desktop/) that uses a wallpaper image as a jumping off point and builds a whole experience around it. It's sort of like a desktop theme, but it really takes it to the next level. You could do something as simple as wallpapers to going for the whole slick UI redesign, and offer these packs as downloads from your site.
As far as automatically loading wallpapers from an external source goes, apparently the application Wally (http://www.becrux.com/index.php?page=projects&name=wally) is the go to app to make this happen, and better still, it's open source. It looks like uploading images to flickr would be just about all it would take to make this work. Fans could subscribe to your user, or album, or even specific tags, it looks like.
2. 3D Printing
I think there is potential for the 3D printing idea to work even though you're a painter, but It would involve some work on your end. This isn't something I've put any thought into before now, so if the idea seems a but raw, it is.
I've noticed there is a lot of texture in your pieces. If you amplifiied this, you could potentially produce something that looked like an etching of your piece, which could be printed from a 3D printer. If you recommended the use of porous materials in the printing process, someone printing one of these 3D "prints" could then almost do a paint by numbers using acrylics or what have you, using the topographical differences of each area as a guide, and they could replicate (in much less detail) one of your paintings as a fun weekend project, while still keeping the whole thing on rails. This sounds like something that might be more fun for kids (at least the painting part) than adults, since it would produce something of far lower quality than your orginal, but it's still something that might help build a community around your work, and build future interest, especially if the process of creating the 3D model was not prohibitively complex.
Mike Krahulik of Penny Arcade (penny-arcade.com) does a regular streamcast of himself drawing one of that weeks comics, and he has two way communication with the audience. Now, I know that you said the instructional videos you've posted do a lot better than posts of you painting, but it seems like a mixture of the two could get you major traction, especially if they were live, and the time you paint is pretty regular (I don't know how long your projects tend to take, but I'm not trying to suggested blank canvas to completion in one go). If you made yourself available to your fans while painting, and then also posted the video/s of the project after the fact, it might allow fans to go with you the entire way, while asking pointers, or asking you to go over some specific technique in more detail (you could have a scratch canvas set up nearby for this possibility). Again, as others have suggested you could also take suggestions, with your viewers voting on the spot. At the end of a project, you could then (another suggestion from abve) time lapse the entire painting by stitching together the various videos of each streaming session. All of these things working together could really help in creating not only fans, but a sort of audience, turning your process into a series of performances, and the overall process into another separate interest creating good.