I'm just starting out with extension development and I believe I've read a good portion of the documentation, but I think I'm missing something very obvious.

To start, I am trying to pull the source code for any site, so I'm using browser action. Eventually I want to pull the image URLs from the source code, but I need to figure out how to get the source first!

Please help! Thanks.

So far I have in manifest.json:
"browser_action": {
	"default_icon": "icon.png",
	"popup": "popup.html"
and popup.html consists of

  chrome.browserAction.onClicked.addListener(function(tab) {
    // We can only inject scripts to find the title on pages loaded with http
    // and https so for all other pages, we don't ask for the title.
    if (tab.url.indexOf("http:") != 0 &&
        tab.url.indexOf("https:") != 0) {
      //executeMailto(tab.id, "", tab.url, "");
} else {
      chrome.tabs.executeScript(null, {file: "content_script.js"});