I know one can transfer pictures from a media card onto a computer. Can one also transfer pictures from a computer to a media card? If one can how do you do it?
Yes you can. Treat the media card as if it was another drive. You can copy to, or drop files. It may not be possible to view such files on your digital camera after you put the media card back into it; that may be different for different media cards and/or cameras. It also may screw up your camera overhead file, so try it out when there are no image files on your card or at least ones that you don't mind loosing if something goes wrong.
Actually you can get the camera to see modified JPEGs after transferring them back. But you need to use a small program called TVwriter. It was part of our article on using the Casio Z55 as a portfolio to show your images. Here's a link to our article.