They wore it, but for like 1 minute to take the picture for the card
In addition to the picture, the girls also decorated the shirts. You can see pieces of their drawings in the costume bits in the cards.
A lot of people question if these costume cards are genuine. I'm convinced that that they are real.
1. The clothes are specially selected for a certain photoshoot. I mean, it's not their actual stage/concert outfits or their personal clothes, so it's expendable. In some cases they re-use the photo's for the card collection, a PB, DVD and sometimes even a calendar... it's not like cutting up a couple of dresses is a big expense. In other words, they have no reason to fake it.
2. If it ever got out these things aren't real it would destroy the market. They would be stupid to take such a big risk.