To change the screensaver on Imac, we should follow these steps:
In the Apple Menu, go to System Preferences to open the window where we can change the preferences for system options.
Choose Desktop & Screen Saver button to display the preferences of the desktop and those for screensaver.
Click on Screen Saver tab to modify only the Screen Saver preferences, and you will see all the installed screen savers in the left, on a panel.
In the right side, there is a preview tab.
Choose a Screen Saver from the list on the left side, the one you would like to use.
You can test it also to see how it would look in use.
You can also use for your screen saver a slideshow of images you like. To have a slideshow screensaver, do the following:
In the Screen Saver preferences tab window, choose Library and this is how you will transform your iPhoto pictures in a slideshow screen saver. To use your personal, preffered pictures, Select Pictures folder or Choose Folder, go to the folder which contains your images, or input it's path and then click Choose to make it functional.To use a third-party screen saver, just install the screen saver package, and if it doesn't place the .saver file you would like to use in the right place, you must find it and place it in the right Screen Savers folder. Its path is "root of your hard disk ~/Library/Screen Savers". After you placed correctly the .saver file, then follow the steps from the beginning, like you would simply change your screen saver, and the new one will appear in the list on the left panel.
After you picked up your favorite screen saver, you can customize it now. Here are some elements you can change in the Screen Saver preferences window: click on Options to modify specific attributes of the screen saver to your liking (they vary from on screen saver to another), move the "Start screen saver" slide button to change the time after the screen saver should start, if you would like the screen saver to start when you click on a specific corner on your desktop then you should select the "Hot corners" option, and when you will place the mouse in the selected corner, the screen saver will automatically start. If you like adventure and surprises, you may check the "Random screen saver" box and now the computer will choose every time a different screen saver from the installed ones.
Now that you found out how to use screensaver on Imac, you can start customizing it, choosing your favorite images as screen saver or explore the variety of items available on Internet. There are plenty of thematic screensavers, so you can pick a screensaver related to your hobbies, to your favorite singer or celebrity, but also use your personal pictures, to review them each time your computer is sleeping.