function findExistImage(newImageOrSrc) {
    if (typeof newImageOrSrc === 'string') {
        var cachedImgObj = globalImageCache.get(newImageOrSrc);
        return cachedImgObj && cachedImgObj.image;
    }
    else {
        return newImageOrSrc;
    }
}
Caution: User should cache loaded images, but not just count on LRU.
Consider if required images more than LRU size, will dead loop occur?

@param {string|HTMLImageElement|HTMLCanvasElement|Canvas} newImageOrSrc
@param {HTMLImageElement|HTMLCanvasElement|Canvas} image Existent image.
@param {module:zrender/Element} [hostEl] For calling `dirty`.
@param {Function} [cb] params: (image, cbPayload)
@param {Object} [cbPayload] Payload on cb calling.
@return {HTMLImageElement|HTMLCanvasElement|Canvas} image