SECURITY_ERR: DOM Exception 18 on using getImageData in a Chrome Extension

I'm writing my first Chrome extension. I'm trying to use jQuery and the jQuery Image Desaturate plugin to desaturate an image on a page on http://www.flickr.com.

I'm loading my script (and jQuery and the plugin) programatically in my background.html:

  // On browser action click, we load jQuery and the desaturate plugin, then 
  // call our own flickrnoir.js to desaturate the photo.
  chrome.browserAction.onClicked.addListener(function(tab) {
    chrome.tabs.executeScript(null, { file: "jquery.js" }, function() {
      chrome.tabs.executeScript(null, {file: "jQuery.desaturate.js" }, function() {
        chrome.tabs.executeScript(null, { file: "flickrnoir.js" });
      })
    });
  });

I've specified permissions for Flickr pages in my manifest.json:

"permissions": [
  "tabs", "http://www.flickr.com/", "http://*.static.flickr.com/"
]

That appears to be working fine, and I can, for example, turn the background of all divs on a Flickr photo page red by adding this to flickrnoir.js, and then opening up a Flickr page and clicking on my extension's button:

$("div").css("background-color", "#ff0000");

...so, I successfully have jQuery loaded and it can successfully access and change DOM elements of a http://*.flickr.com/* page.

However, when I try to use the desaturate plugin to desaturate an image (or all images, in fact) I run across a security error. My code:

$("img").desaturate();

...eventually ends up in the jQuery.desaturate plugin's code running this line:

var imgPixels = canvasContext.getImageData(0, 0, imgW, imgH);

At that point, Chrome throws a security exception:

Uncaught Error: SECURITY_ERR: DOM