The special gallery is for the actual sketches themselves to make it easier to find them. It has been discussed a few times and decided that the cards should be uploaded to the gallery according to stamp set for the best overall access to them. You can always do a gallery search specifically for challenge cards by the keyword, for example SC05 for Sketch Challenge week #5 or LSC02 for Limited Supply Challenge Week #2. Or as was posted earlier you can read through any challenge thread and click on links to cards from there. HTH.