Not sure if it's actually required for you to do so, but...
the easiest way I could imagine would be to just zip the PSD file and post that on the forums
Otherwise you'd have to save each layer as a separate PNG and upload it.
It is required i think. At least according to the part of the rules i quoted below. That is if i am mistaken.
"When you submit your entry, we ask that you either include a PSD or a ZIP file containing all images related to your egg, make sure the images are transparent otherwise they can't be used. In your thread, please include an image of the egg in one piece (including the shading etc). If you have made your own egg from scratch, include an image of that, along with the PSD and/or ZIP file."
Also,
@Cloudyblue somebody explained to me earlier in this thread on how to ZIP your egg layers. Here is what he said below:
For zipping:
Put all of your files into a folder, then (if you're on windows) right click on the file, navigate to "Send to" and then select "Compressed (zipped) folder" - it has a symbol that looks like a folder with a zipper on it. If you're on a Mac, put all of your files into a folder, then right click the folder and select "Compress Items". Take the new file that is created and submit that.
(When you are done with Zipping add the ZIP file to the thread as an attachment.)