This frustrates a lot of people and you are not alone. I Deal with this daily with new converts I train. Many of them Cannot revert back to iPhoto as they had not updated to the last version so it is now greyed out with a white circle and slash. They have difficulty enough with the new look and feel, especially the older users less willing to deal with change, so trying to take them over to Image Capture for a multistep process to accomplish what last week only took a click on a button in the same app is sort of making the Apple experience and simplicity MIA.
The white circle and slash on the icon should not matter, if iPhoto has been purchased from the AppStore or came preinstalled. As long as iPhoto is showing on the Purchases tab of the App Store you can update by reinstalling:
Reinstall as described in this User Tip:
Get iPhoto 9.6.1 if you didn't update before OS... | Apple Support Communities
The main idea is to move the incompatible iPhoto app to the Trash, so Spotlight does not see it as installed, reload the App Store by pressing ⌘R, and then to try to download again from the App Store - from your Purchases tab, not from the main page, since you can neither buy iPhoto nor update it, but you can reinstall, if your AppleID is associated with it.
So move iPhoto to the Trash (don't empty the Trash), relaunch the App Store. The button to the right if iPhoto should change to Install on the "Purchases" tab.
If you do not see iPhoto there, try to unhide your Purchases as described on this page: Hide and unhide purchases in the Mac App Store - Apple Support