Wherever there is a popular game, there is fan art: custom wallpapers, t-shirts, etc.
One thing they usually have in common is that they feature artwork from the game, or something close to it. Obviously, some people are talented enough to reproduce the artwork with their own skills. But I think more often than not, it uses actual artwork straight from the game.
Take a look at some image search results for Portal for some great examples.
I’m a programmer so I understand that for some games, you could literally just lift the artwork right out of the game files. But for things that are dynamic and animated, that’s not really an option (for example, the blue and orange portals themselves, in Portal).
Grabbing the artwork I would imagine has to involve taking screenshots and then manipulating the image to give it transparency. But I’ve tried this, but it seems impossible to get a perfect ‘lift’. Sometimes you can do things in the game that help, like get the subject to be in front of a perfectly even surface for the screenshot (like a green screen). But even that often times won’t turn out well due to anti-aliasing, etc.
Are there better techniques? Are there any tutorials out there for Photoshop (or any other tool) specifically for this kind of task?
It sounds like you need to do defringing when you’re “lifting” the graphic from in-game to a white/other background. If that’s the case you should check out the answers to one of my questions: How can I fix the edges of foreground objects pasted onto a new background?
If you’re getting aliasing all over the images, rather than just at the edges, try increasing your screen resolution and graphical effects before taking a screenshot. Your example, Portal, provides a lot of options for this. It may slow the game down if your computer can’t handle it, but that’s OK if you’re taking screenshots and not actually playing the game.