Open TwOrig Image

event.js

var isCreatedContextMenu = false,
    CONTEXT_ID = "OpenTwOrig";
chrome.runtime.onInstalled.addListener( Run );
chrome.runtime.onStartup.addListener( Run );

function Run()
{
	if( isCreatedContextMenu == false)
	{
		chrome.contextMenus.create
		(
			{
				"type" : "normal",
				"id": CONTEXT_ID,
				"title": "元画像を開く",
				"contexts" : ["image"],
				"targetUrlPatterns" : ["*://pbs.twimg.com/media/*"]
			}
		);
		isCreatedContextMenu = true;
	}
}

chrome.contextMenus.onClicked.addListener
(
	function ( info, tab )
	{
		if( info.menuItemId == CONTEXT_ID )
		{
			Download( info.srcUrl );
		}
	}
);

function Download( src )
{
	var a	= document.createElement( 'a' );
	a.href	= ReplaceWithBiggest( src );

	a.target = '_blank';
	a.dispatchEvent( new CustomEvent( 'click' ) );
}

function ReplaceWithBiggest( src )
{
	if( src.indexOf( ":orig" ) != -1 )
	{
		// none
	}
	else if( src.indexOf( ":" ) != -1 )
	{
		var srcSplitColons	= src.split(':');
		src	= srcSplitColons[0] + ":" + srcSplitColons[1] + ":orig";
	}
	else
	{
		src	= src + ":orig";
	}

	if( !GetSrcExist( src ) )
	{
		src	= src.replace( ":orig", ":large" );

		if( !GetSrcExist( src ) )
		{
			src	= src.replace( ":large", "" );
		}
	}

	return src;
}

function GetSrcExist( src )
{
	var xmlHttpRequset = new XMLHttpRequest();
	xmlHttpRequset.open( "GET", src, false );
	xmlHttpRequset.send();

	if( ( xmlHttpRequset.status == 200 ) || ( xmlHttpRequset.status == 500 ) )
	{
		return true;
	}
	return false;
}
// EOF

manifest.json

{
   "background": {
      "persistent": false,
      "scripts": [ "event.js" ]
   },
   "description": "twitter.com のツイート内画像を右クリックした際に、「元画像を開く」メニューを表示します。",
   "icons": {
      "128": "icon/icon128.png",
      "16": "icon/icon16.png",
      "48": "icon/icon48.png"
   },
   "manifest_version": 2,
   "name": "Twitter, Open Original Image",
   "permissions": [ "contextMenus", "*://pbs.twimg.com/*" ],
   "version": "1.0.0"
}