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Just refer this link to set some images from your Picture folder as screensavers
For removing the image that you have now
go to "/System/Library/PreferencePanes/DesktopScreenEffectsPref.prefPane/Contents/Resources/DesktopPictures.prefPane/Contents/Resources/Collections.plist"Just remove the section with the image name that you wish to remove and save the file in the same location.