I saw someone once was selling a SSX 3 Allegra poster on e-Bay, though they wanted a lot for it. The poster was similar to the SSX 3 box cover art. Might still find something there if you wanna look (of course, checking for item validity is always a must
). In addition, would any store near you have used SSX 3 displays you could use (like giant versions of the box art, life sized Allegra cardboard displays, etc.)? You might be able to get such rare items by talking to the store mangager/owners.
I rather like Krystony's suggestion of using the group anime picture scene in the game. The idea of the SSX 3 mountain is pretty good, too. Trick will be in getting an image large enough or at a good enough resolution which will enlarge nicely into poster size without becoming pixilated.
Another idea that I just thought of would be to make a mosaic photo using many, many tiny screenshots from the game (and perhaps elsewhere) that, when combined, form a larger picture. You've probably seen such composite mosaic images before, though such a project would take a lot of planning and a tremendous amount of time to construct.
If you're willing to spend more money, you might also consider commissioning someone to make a poster-sized SSX 3 themed pic for you - or you could attempt to create one yourself if you're feeling artistically inclined.
Good luck with the creation of your shrine! Looking forward to seeing how it turns out.
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Incidently, are you still looking for an Elise sig, Slacker? I made one a little while ago and (hopefully) still have it posted successfully in the "Sig Workshop" thread.