When saved as PNGs, semi-transparent images (such as those that would come about from saving a rotated image) are automatically 32bit. You don't have to "set" them as 32bit anywhere in photoshop. in fact, best if you don't mess with any settings that change the colour depth (perhaps in Photoshop the mistake you are making is going to Image->Mode and setting it to 32bits/Channel, which you definitely should not do), just save them as pngs. When importing those images into AGS, make sure to select the option to import the alpha channel as well, and your images should be imported as they are.