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I was trying to change Chrome extension icon dynamically by following this documentation.
Unfortunately, it is not working with the following code:
chrome.browserAction.setIcon({path: 'my_icon.png'});
In console, it is failing with following error:
Unchecked runtime.lastError while running browserAction.setIcon: Icon invalid.

After reading elsewhere on the web, I found that we need to specify images with size either (or both) of 19x19 px or 38x38 px.
So I resized my icon image, and made 2 copies of it as follows:
my_icon-19.png
my_icon-38.png
Now when I tried the following code, it worked as expected:
chrome.browserAction.setIcon({ path: { "19": "/images/my_icon-19.png",
"38": "/images/my_icon-38.png" } });
You do not need both of the versions to make it work, so following would work as well:
chrome.browserAction.setIcon({ path: "/images/my_icon-38.png" });