Recently I received a Nexus 4 device. I logged in into my Gmail account and it popped me a list of what all from Google Account should be synced to my Nexus 4 device. I accidently also asked the device to sync all the photographs from my Google account to the device.
A few days later I realized this and I wanted to remove all the photographs that were synced. I disabled the photo sync in my account on the device and went to the gallery app to manually remove the photographs. But what!!!
There was no option that would let me delete the pics from my Nexus 4. I quickly searched the web to figure this out, I found a couple of links but did not find it useful. Many people were getting confused from deleting a photographs on the device versus deleting the synced photographs.
Here is how I figured out how to remove the picasa photos from Nexus 4.
Settings -> Apps -> All.
Scroll down until you find the Gallery app in the list.
Click and open it.
It will show you something like this.
Just click on "Clear data" and all the locally stored Picasa photos will be deleted from your Nexus 4 device.