Icon image does not show in Dock
I have a Mac mini 2011 and I have recently upgraded to Yosemite and all of a sudden and for no apparent reason some of the icon images in my dock appear invisible. The application is there, I can click it and it will open, what I can´t see the icon image. An empty space is left in the dock between other icon images that do show. This happens with the icon image of Skype, Word, itunes, Appstore and others. How can I restore the icon image to my Dock? I already tried removing the icon from the Dock and putting it back but it did not work. Thanks for any help.