A conversation about this had started in The Giant Art Thread. I had a good look through the images yesterday hoping to find the source image for the Neo Premium File Pichu but couldn’t find it, however sozaijiten does appear to be part of a wider set of stock images available on designpocket.jp - it wouldn’t surprise me if that Pichu background is in there.
Interestingly they did still appear to be using this set of stock images in 2009:
What I find really great about these is that the stock pictures have descriptions which give a hint at where they were shot. The above Pichu card background is シュレックホルン (Schrekhorn); a mountain in the Swiss alps.