You would have to post your photo to some image server on the web. This isn't a Mac issue. Even PC users will need some place to post theirs. To capture a screenshot on a Mac, use Apple + Shift + 3. Should save an image of your desktop right on your desktop as a pdf file. Take it to Photoshop or some other image editor and resave as a jpg file. Then upload to some server somewhere and post the link here.

As for my desktop, I have the settings on my Mac configured to cycle through a different desktop each time it is either awakened from sleep mode or each time I reboot in the morning. Most of my desktops are 3D shots taken from MeCompany. Cool site that is responsible for some of the artwork for Bjork's albums.