For Mac Users
To save a picture on your computer, it might be necessary that you press "ctrl" and click your mouse or touchpad while your curser is over the picture. That opens the menue, where you can choose the according option.
Lean back and have a glass of wine.
Send an Email
If you have saved a picture on your computer, you can easily drag it into an opened email with the typical drag-and-drop function.
Print a Postcard
I am able to print cards of good quality, although the resolution is limited to about 1280x840 pixels, in that I use inkjet paper
(e.g. Photo Rag) and long lasting Ultrachrome type ink on an inkjet printer and set high-quality printing in the printing dialog.
But you also may get the card printed in a shop.
The resolution is a test. I might change it, if it is not sufficient.
Set as Wallpaper
How to set a picture as wallpaper depends on your browser. For instance with Firefox right-click on the picture and "Set as Wallpaper". Downloading (using "Save") would be an alternative. You have to try with your computer.