manifest.json
    "permissions":
    [
        "storage",
        "webRequest",
        "webRequestBlocking",
        "http://*.gelbooru.com/*"
    ]

Javascript Code:
    var handler = function(details) {
        details.requestHeaders.push({
            'name': 'Referer',
            'value': 'http://gelbooru.com'
        });
        return {requestHeaders: details.requestHeaders};
    };

    var requestFilter = {
        urls: ["http://*.gelbooru.com/*"],
        types: ["image"]
    };

    var extraInfoSpec = [
        "blocking",
        "requestHeaders"
    ];

    chrome.webRequest.onBeforeSendHeaders.addListener(
        handler,
        requestFilter,
        extraInfoSpec
    );