Chrome extension: context menu icon not showing

I am building a Chrome extension and I set up a context menu which will show up and check the items (images or text) that the user selects from a table in the extension’s popup.

This is my code for creating the context menu in popup.js‘s window.onload:

chrome.contextMenus.removeAll();
chrome.contextMenus.create( {title: "Toggle Selected",
  documentUrlPatterns: [ "chrome-extension://*/popup.html"],
  contexts:["selection"], onclick: toggleselected} );

This is the function that gets called on the context menu’s onclick – also in popup.js (simplified version):

function toggleselected(info, tab)
    {
    var selection = window.getSelection(),
            q0 = document.querySelectorAll("[id^='id0']"),
            q1 = document.querySelectorAll("[id^='id1']");
    for (var i = 0; i < somearray.length; i++)
        {
        if (selection.containsNode(q1[i], true))
            {
            somearray[i].checked = !somearray[i].checked;
            q0[i].checked = !q0[i].checked;
            }
        }
    updatesomearrayinfo();
    }

And this is my manifest.json file (again, simplified version):

{
"manifest_version": 2,
"name": "Extension Name",
"version": "1.0",
"description": "Random description",
"browser_action":
    {
    "default_icon":
        {
        "16": "icon16.png",
        "32": "icon32.png",
        "48": "icon48.png",
        "128": "icon128.png"
        },
    "default_popup": "popup.html"
    },
"background":
    {
    "scripts": ["somescript.js"],
    "persistent": true
    },
"permissions": ["contextMenus", "tabs", "downloads", ""]
}

Everything works fine and the context menu correctly shows up when needed ... except that it doesn't have the extension's own icon next to it, but the default Chrome extension icon (the grey one). What can be done to correct this behavior? The icons in my manifest are ok, one of them (the 16x16 pixels, I believe) is correctly shown on my extension button, and apart from others usually doing the whole context menu thing from event pages, using chrome.runtime.onInstalled, contextMenus.onClicked.addListener and other listeners (which I am reluctant to use, since I specifically need to work with the variables from popup.js - e.g. somearray and queries on popup's elements), I don't seem to be doing anything different than things done by other extensions or the code samples in the answers here on StackOverflow. And they all get their own extension icons shown, apparently without doing anything special.

EDIT: Also, does anyone knows why the context menu for a selection is triggered only when the selection contains more than one specific item (an image, in my case)?