OSX 10.10.3 Update - App Store icon image missing
https://discussions.apple.com/thread/6983727

Resolved it from this workaround:


Click informations on the blank icon.
Click on the library icon in the left corner of the informations window and copy it.
Open Preview.
Paste the copied icon and save it on desktop.
Now, just drag and drop the new icon to the old icon of the informations window of the broken Library icon.