Can I transfer photos from computer back onto camera?

I'm using iphoto on a mac and a Nikon D3000. I was wondering if it is possible to transfer photos that I have on my computer back onto the memory card that I accidentally deleted them from.

Rarely.

If you have made ANY changes in the image file, even rotating it, your camera will NOT see the image file.

Your cameras LCD is there to allow you to review your shots just after you take them to help you determine if you need to reshoot or not. It is NOT an image viewing device for you to use to show off your images on the camera

This means that even if you were to place the image files back into the same file folders on the cameras memory card, your camera will probably NOT see them

You can transfer them, yes, no problem. Use Finder. One note though. If the pictures have been manipulated by the computer in any way, those pictures will not come up in Preview mode in the camera. The pictures will be in the card but not visible in Preview. You can use the memory card to transfer the pictures to another computer and preview them there (in the other computer) or have them printed in a photo printing shop.

Nope, and why would you want to?