I think you just have them labeled as "icon" and "info" in your world folder, and they should work... I think...though, I don't know if it has to be a certain type of image (if so, perhaps .png, like custom objects).
Edit: Yep, it works, and yes, it's a .png image. Rename the picture as icon or info, respectfully, and move it to your world folder. From what I saw, it appears to have reset it back to the standard info/icon picture in the folder, but if you play it in the game, the picture is the one you want.